There are people who are turning mid 13's and do not use a AFPR. We already are running approx. 52psi from our fuel delivery. The SC 3.8 engine is not like the GN 3.8 where fuel delivery was a problem.
As far as voiding your warranty, I only have 9000 miles on my car and my service tech is aware of the mods. GM has to prove that the mods you put on a car are a direct cause of something failing.
So you don't have to wait to you get miles on the car. All the mods I have are considered superficial and do not degrade the car in any way.
As far as going to a 3.4" pulley, there is not much noticeable difference from a 3.5". These pulleys are undetectable from stock. By the way, the stock size pulley is 3.8".