Yeah, Twins never hit their advertised theoretic and neither does a single, although it does come alot closer, there are always loses, heat,drive train etc etc.
Are you talking about mercury, I was there for that dyno day a couple of mates had their cars run up.
I made 431HP on R33 N1's on my 32 at ~17PSI I would like to make more then another 36HP with a few hundred horsepower more of turbo on board.
Not to take anything away from that result, because that thing would go like a bat out of hell !
My N1's are busted now (oil seals) so -5's will be going on and shooting for 400AWKW.
See how I go i guess.