From what i know about turbos, a 7psi boost should increase power anywhere from 40 to 50 percent. The lower the compression ratio the more boost you can run safely. But recently i've heard that the higher compression motor running lower boost levels is a better bet, when not under boost the motor isnt sluggish and the higher exhaust velocity spools the turbine quicker. My Syclone has a turbo and when i had the motor rebuilt with forged pistons and rods, i had the compression reduced so i could run higher boost levels(which i never did), now the truck doesn't feel nearly as quick as it once was with the same 14.7 psi.
My only experience with turbos comes from my v6, so whether or not any of this holds true to a v-twin I'm uncertain.
As for blow off valves, they do sound kind of cool but without it sounds just as cool i think.