2.55 upper is 13psi. proabably ~550-560whp.
most people don't do just the upper alone. some people have done just the lower 10% pulley which was 540-550whp. that is 10.5psi or so
my .02 if your pulling the snout to do the upper port it and throw a TB on at the same time.
then when you get done with all your mods you will be in the 600-625 range you will need injectors too at that point as you will be out of duty cycle.
2013 ZL1 -sold
2013 Megacab 8" lift 38"mt's