You should pick up anywhere from 5 - 15 rwhp (way more if you ever go with any type of forced induction) and the gains will be across most of the powerband.
Roy (user Roy on Mach1Registry and here on SVTPerformance) ported a Mach intake for me a couple months ago and it made awesome gains over my old Aviator manifold (30+ HP/TQ)! The car laid down some really good numbers for a stock engine bolt on car too, 344/343 SAE, 354/353 STD.