I have a 1988 Trek 520 that is an awesome bike, but recently it has been raining a fair bit and the brakes are downright useless in the rain (I have fairly new Koolstop Dual Compound pads). I have long been considering a swap to mini-v brakes, but have recently wondered if I should go ahead and swap to 700c wheels too. It is my understanding that the old steel rims also contribute to the poor rain performance of the brakes. Furthermore, I will be moving from the Bay Area to Denver soon, and wonder if switching to 700c would allow me to put slightly wider knobby tires on for the winter. Any thoughts or advice you have would be excellent!

Number 1 is replace the steel rims. And what on earth would install 'steel' rims on a Trek 520?

I would certainly consider switching to alloy wheels for better stopping and better overall ride quality. However switching to 700c wheels may not be practical. You might get lucky and find some brake that works but I believe in general swapping 27" to 700c wheels on a bike with cantilever brakes isn't always practical.

Swapped three bikes, two being mid '80's Treks, from 27 to 700c (and one of them back to 27 when a tasty set of wheels came along) without much in the way of trouble.
Also, are you sure you have steel wheels? That would be unusual unless someone swapped them on in the past. And as said above, ditch 'em for a set of alloys and ride it. Then if you still think the brake performance is poor, look at other brakes. Blaming the brakes for poor performance while using inferior rims is pointless.

Do you have a picture of the wheels/brakes set up on the bike right now? That would help us determine if the wheels are indeed steel (some deranged person must have installed them prior to your ownership), if your brakes are set up as ideally as possible, and what kind of clearances we're working with. I have thought about and tried the canti-to-V brake conversion on a few touring bikes, and it comes down to whether or not the canti posts are wide enough apart (80-85mm ideally) and low enough (short distance from them to the dropouts) to facilitate brake type flexibility.

I'm currently yelling happily from the rooftops about Tektro CR720 canti brakes on my 1990 touring Cannondale. Canti posts are 80-83mm apart, and measure 290mm from the dropout/wheel axle to the center of the post front and rear. The 720s just fit, but V-brakes certainly do. What was nice about the 720s is that they look great, are immensely strong, and with Kool Stops, stop with as much authority (in wet or dry) as the best dual pivot side pull calipers that I've ridden (on my other bikes). Oh, and they DON"T SQUEAL! Seattle is very hilly, so if it passes the test here, it'll pass anywhere else.

Your canti brakes should easily accommodate going from 27" to 700C. The stiffer rim construction (most 27" wheels are "single wall" vs. "dual wall" for 700C/newer wheels) will help, and if you can find 700C rims that have machined brake tracks (small grooves), that helps brakes bite even better.

As per usual, proper setup, new/smooth and easy functioning cable action (good cables and housings), proper lever setup/positioning, and a good brake lever are a must to get good performance. A picture and report of that would also help us to get the whole picture of what's going on. You have a good bike, let's get it going properly.

Steel rims should never have been put on that bike; it came with perfectly good alloy ones. Like nesteel says, you can often swap in a pair of 700c rims without much trouble. Sometimes it means having to change the calipers; sometimes you just have to adjust the brake pads. On my Cannondale, I got away with it by using a slightly narrower rim than was on the bike before.

Unless you can find someone here who's done the swap to 700c on that particular year and model, you'll kind of have to fly blind on what calipers will work with the slightly smaller wheel set. But again, maybe the ones you have will be fine.

Once you've got that worked out, modern compound brake shoes are always an upgrade, but again, that won't be all that noticeable on wet steel rims.

The brake calipers are almost never the problem. OTOH, my V brakes always seem a little grabbier than cantis, so they might help out once you get some proper rims.

I don't know if the OP is still on board here, but changing to 700c will change the pad-dive angle considerably, and problematically so if the rims are on the narrow side.

What this means is that the pads will be occupying a vertically wider swath of the rim's side wall (or braking track). This is more of a problem with 1) rims having a narrow brake track and 2) rims that are much narrower than the tire (making the pad's more-vertical travel path possibly cut into the tire sidewall as the pads retract).

I've found even the Trek touring bikes to have very little clearance for fenders or even a tall tire, so 700c gives 4mm of additional vertical clearance vs. 27" assuming the tire cross-section is kept the same.

I've put quality Araya and UKAI steel rims on bikes that came with alloy rims, not a problem if it is for dry-season riding and using correct brake pads.
The OP needs alloy rims though.

Make sure that the pads are contacting the rim squarely such that after initial contact, the pad does not tilt as lever force is increased.

The lever shouldn't feel spongy after the pads contact the rim.

It would't hurt to take a little sand paper or emeryboard to scuff the pads and even some steel wool on the rims then rubbing alcohol on the rims to clean all the dirt and grit off.

Going to 700c wheels will allow for fatter tires and/or studs or knobs which wil help with snow. However, usually the snow in Denver is gone fairly quickly so what I have are multiple wheels one pair of which have studs and one pair without. On those days when the roads are icy studs are very nice but a lot of drag when on dry pavement. In warmermonths you might have some gravel grinders and some road slicks as your set. One of the things I love about older bikes is their ability to run larger tires.

Just a note: after 12 years of fighting with the mini-Vs on my Miyata Triplecross I'm switching back to cantis.

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I converted my Raleigh Aleyska to 700c to fit 38's with fenders and the stock canti's worked. Luckily they did since the pivot spring is on the wrong side - you'd either have to make an adapter (I've thought of doing this) or buy Paul brakes in order to upgrade. There's no hills here so marginal brakes are totally acceptable.

I can attest to this fact. I rebuilt a very scruffy '84 Univega touring bike recently, and I had a helluva time finding front cantilever brakes that would work with 27"...eventually went with mini-V's that gave me a lot more room to adjust than most canti brakes. 700C wheels wouldn't have worked at all, unless I went with long side-pull brakes...tried a wheel off my old Raleigh, and the canti posts weren't even remotely in the right spot for 700C's.