ah, i see the straddle cable is not adjustable. that's easy then: lower the yoke, get a shorter double-ended cable or a clamp-on cable end (easy to find these for bmx u-brakes), then slide the posts in so the pads are closer to the rim and angle the pads so they hit the rim squarely. the only problem here is a lack of an adjustable-length straddle cable.

Here's some clamp-on straddle-cable ends that will let you adjust the length of double-ended straddle cables:



That would work on his back brakes to shorten the straddle-cable and lower the yoke. I advise against the use of these; only as a last-resort effort to shorten the straddle-cable. That's due to one more clamp-bolt that may not be tightened properly, leading to an additional liability issue with brakes. Best to get a shorter double-ended straddle-cable that places the yoke about 1" from the tyres.

The front will also need longer brake-cable as I don't see any excess length that can be used to slide the yoke down.

I would suggest using link wires instead of a straddle cable to lower the point at which the wire splits. They can be had on ebay for a couple dollars and are safer. Lowering the point of the yoke as AEO suggested will provide a lot of improvement. Think of the straddle/link wire making approx. a 45 degree andle between the calipers. Also, as fietsbob suggested, different style calipers will improve leverage greatly and can often be had on the bay for reasonable prices. I have three bikes with cantilever brakes and I am switching out the calipers on the one I recently got that is set up just like yours. I use XTR (not cheap) and XT calipers (reasonable). That is a nice bike and you should be able to make it respond like you want. You can also consider V brakes as a replacement if you want to - also available at reasonable prices. Others here have provided many good suggestions for you.

Link wires rely on the ability to clamp a cable at one of the brake arms, which the OP can't do due to the design.
He'd still need to use a clamp-on cable end to pull that off, in which case he might as well make his own straddle cables and retain more of the original design of the bike.

While I fully agree that they don't look all that reliable, I've used those on a bike for years w/o any issues. But sure, I prefer the bought stuff, or failing that, the cable ends I've made myself and soldered in place.

I'm thinking about replacing my drops with bullhorns and TT levers. If I just took my bike to a LBS and told them to install the levers and adjust the brakes, how much should it run me? I don't have the tools/patience to do these adjustments myself...

First of all, the cost of that would be entirely dependent on the quality of the parts that you'd want to use - so any estimate would have to cover a rather wide range.
Maybe $150 and up.
Secondly, bringing a nonstandard bike to a shop and ask them to "adjust it" is asking for trouble and not really fair to the shop either.
The steps needed to get these brake working OK are way off the beaten path for what's covered by an "adjustment" - i.e. replacing the straddle cables.
Don't think there are many stores that would have a well-assorted stockpile of those in different lengths at hand.

I suggest that you get the straddle cable issue out of the way first, then you can ask for a tune.(having removed the biggest uncertainty.)
Or ask for the cantis to be replaced by mini-Vs while you're at it, and it'll a be a straightforward build again.