how much do you gain with 202s over 188S?

IMO, the 2.02 valve becomes advantageous as soon as a valve job that uses a throat cut is used. I've not seen any drop in low end when the proper parts are matched, and typically on a "stage 2" porting job I'll run 2.05 intakes with no noticable loss. But the entire port, as factory machined and cast, will respond better with the 1.88 valve in it. Once you begin to remove the inherant "factory stock" issues, a larger valve's plusses outweigh the negatives. I would say it's impossible to predict how much of a gain because so much of the total output is based on how other parts react in conjunction with others.