I know these for a fact:
- '92 and '93 NA brake calipers, pads and rotors are interchangeable
- '93 turbo rears use the same pad shape, but possibly a little thicker
- '93 turbo rears have a larger diameter, thicker rotor that won't fit in the NA calipers without an adapter to move the caliper out and thinner pads (the pads are like
2 mm too thick)
- '93 turbo fronts are a two-piston design with larger diameter, thicker rotors
- '92 NA 14" wheels won't clear
- My 16" aftermarket wheels clear fine, but the valve stem sticks straight down and rubs the calipers; before actually driving the car, I'll need to get elbowed valve stems; to run 15's, the stem would probably need to be offset to the outside so it completely clears the caliper
I've been told by enough people that I think these are true:
- Early turbo rears are identical to NA rears (therefore, anything true about late turbo rears vs. NA rears is true about late turbo rears vs early turbo rears)
- Early turbo front rotors are the same as NA front rotors, but the calipers are a two-piston design unlike the NA's one-piston design (therefore, anything true about late turbo front rotors vs. NA front rotors holds true here)
- Late turbo fronts have larger diameter, thicker rotors than early turbo fronts
- Late turbo calipers are all bigger and more massive--this makes them heavier (I think I measured about 6-7 lbs per corner for drained caliper + rotor, but don't quote me on that) but more resistant to brake fade
I paid $600 for a '93 turbo conversion, but did not get the master cylinder, proportioning valve or brake booster. I bought mine used from a board member at the forum-that-shall-not-be-named-lest-it-get-edited-out-and-make-me-look-like-a-tard. I also just bent my N/A dust shields a little and they clear fine.
Gairloch: There is a very significant mass difference between the stock rotors from NA to late turbo. I'm fairly sure the early turbo rotors are the same as the late turbo rotors, which means the calipers aren't the only major point of improved heat soak.
Like you said, the biggest advantage is heat soak, however, there should be a significant increase in braking torque for really wide, sticky tires. I haven't driven mine yet, so I don't know.
From other discussions, it seems like the best bet would be upgrading an NA to early turbo fronts, master cylinder and proportioning valve, or to go all-out and get a Wilwood upgrade or something to get late turbo braking in a lighter package. In either case, you should get Dot 4 or convert to Dot 5 fluid (higher boiling point, so heat soak isn't as big of a deal), steel-braided lines (more efficient pedal pressure to wheel torque conversion), and a good set of pads (stock brakes + good pads > late turbo brakes + crap pads). Only if you are experiencing heat soak will the late turbo brakes benefit you (and you might even lose speed because of the 30-ish lbs of extra unsprung weight).