Ok my .02 on the subject:
I have an 85mm C&L on my 02 GT it was running pig rich from ford and really leaned my tune out and I'm sure I gained 5RWHP or so. At the time before I knew any better I thought it was a great Mod.
Now the problem as I've learned a few things. I had my Mach 1 in for a baseline dyno and thought I'd put on the 85mm C&L on with the Mach's electronics my last pull since I had it.
The problem was my Mach had a very good tune from Ford and was running an A/F of 13 across the RPM range. The Mach also uses an 80mm MAF and I thought it would maybe lean the car out to 13.5 to 13.8....WRONG!
It leaned it out to a dangerous A/F of 15! Although I didn't notice any detanation it really freaked me out.
The way I'm leaning now is the MAF is really a useless mod, it's big enough. If your lucky enough to be running rich it will help but it's a big shot in the dark and could be bad for the life of your engine.
If I were to do it over I'd just buy a tuner like the Preditor to modify my A/F curves, be able to correct for gear/tire sizes, troubleshoot codes on my car,
adjust timing, remove the rev limiter, and the govern for top speed. Of course you'll probally be out $50 to $100 dollars more and some dyno time but I'd sure think about it.JMO
edit:
BTW with the 85mm the Mach gained a massive 1/2RWHP and 1RWTQ