Let me elaborate my story a little bit...I do the same thing, my exit ramp is even less then 100 yards from my house and I come off the freeway everyday doing 70 mph. I brake how I want to, sometimes hard or soft, whatever the situation warrants, and i've never had this problem, and mines an 01 sport with 20's on it.. I'm confident that this wouldn't cause any problems with rotors warping. There would be a ton of people complaining about this problem and this is the first time i've ever seen anyone mention it relating the 2 things..

Also if you drive and stop aggresively (which it sounds like you do, no problem there but just saying) I would suggest upgrading to the Brembo big brake kit all the way around (yes $$ i know) or like you said going up to a 4.6 or 4.8 brake upgrade..