i currently ride a 58cm (58cm TT) with a 90mm stem.
i'm selling a frameset and replacing it with a new smaller frame - looking to try for some more saddle-bar drop and riding with a longer stem, since i hear that it improves handling. i'm plenty flexible.

a 56 with a 110mm stem is probably where i would start.
what do you think of a 55 or 54, assuming that the contact points will be set up the same?

Assuming you can get the contact points the same, it will feel and handle a little different, but should work. Just make sure you have room to compensate for the steeper angles. Saddle setback, etc.

The thing about 54, 56 and 58 frames, is that most manufacturers make a significant geometry/sizing leap between two of those sizes, but not the other. For some brands, the 56 and 58 are within .5cm in stack with the same reach, while the 54 might have a 2.5cm shorter stack. Often, there are marginal differences between 54 and 56, while it's the 58 model that has significantly higher stack. Depending on how close to the frame's limitations your saddle to bar drop needs are, some brands may not be able to provide an acceptable range. The nice thing is, that if you need a significantly larger drop, and your current reach is correct, you have a nice range of stem sizes that will allow for some flexibility in reach, since you can compensate for a shorter frame reach with a longer stem to a point.

How much drop do you have currently? With what stem angle/how many spacers?

How much are you predicting you will need?

This is not a very good question to ask on an internet forum. You need to go down to you LBS and climb on some actual bikes to feel if it is right or not. Also, bring along someone with you that can look at you on the bike to say if it looks right.

obviously this will involve trying out the bikes, not just randomly buying. i don't mean for this to be a "WHICH BIKE SHOULD I BUY" thread. just looking for some pointers - like Rat Fink's post. that's good stuff.

i have some decent drop now, but i could go lower. even if i end up not going much lower, i would like to get a setup that lets me have a longer stem.

when i first got set up with 58s, i was very new to cycling. now i'm a lot lighter and more flexible; it's time to get re-fit and try something new. i am thinking CAAD 10 or similar light stiff alu frame.

this is my current setup: https://www.flickr.com/photos/7380051@N03/5675478783/

Just make sure your knees wont hit the bar when you're out of the saddle. I was fine on my 58cm with a 115mm stem when I climbed seating most of the time, but not I have to make sure I don't hit the bars w/ my knees. A smaller frame with a setback saddle might be fine seated, but not work so well standing.

i stopped by a shop today to return something and looked at the CAAD10 - the guy there was like "definitely 56." and that nobody should ever use a 90mm stem if they can avoid it.

this is EXTREMELY frustrating.
you're supposed to trust bike shops - which i did when i got my first road bike. the guy said "i'd go with the 58." did not say anything about how smaller frames and longer stems are preferable. so i got used to the 58/90mm....and i do love it. i'm just sick of people looking at me like i'm crazy for having that setup.

go to sid's bike shop. One of the nicest and most knowledgeable people around. Their fitters are very patient (spent 80 minutes with me) and the rates are quite reasonable, too.

personally, i ride with a 135mm stem on a 52.5cm TT whereas a 54.5cm stem with 110cm would be recommended. But a 54.5cm frame would have a HT that's too tall, so alas i went with 52.5cm

figure out what your ideal toptube + stem measurement is, and buy according to that. even though i could hypothetically fit your 58cm bike french-fit style, i'd prefer a smaller stiffer frame with more seatpost and a longer stem. short stems lead to uncomfortably quick steering, i prefer something in the 110-130mm range.

Nothing wrong with a 90mm stem. And you seem happy with your bike. It really doesn't matter what others think.