I have a nice customized plenum, and Andrew at Pro auto said that cars with almost identical mods without the plenum and TB(Q45) typically made less power, and he said it definatly improved flow. mine has a 35r with a .82r
I also run a gizzmo thermal gasket, to stop heat transfer from the head to the plenum, and the plenum stays quite cool.
so there are gains to be had, I did it as a sort a package, highmount mani, 35r, plenum, TB etc.
shorter piping is always a bonus too. I think I reduced it by a metre all up.
and it looks mint =D
I wouldn't be tempted by a cheapo job tho, I'm not a chinese hater but its one thing you dont want to fail. pain to get on and off too. I still had to make a throttle cable bracket and also Mod the water temp senor ports as when the plenum was built it was dummied up on a head with no VCT sensor!, 10 hours of fab later lol. was worth it in the end tho.