well regarding the rx8, i had at one point thought of buying a typhoon K&n induction intake to replace the standard air box in mine.
however due to its positioning in the rx8, warm air would be sucked in rather than cold air as the cold air travelled down the pipe
situated in the front bumper area into the air box and stays cold.
the typhoon intake was sucking in warmed engine bay air. the result of this is actually less power than say a standard air filter/air box.
ok the typhoon sounded better but actually reduced power.
the guy i bought my rx8 off wanted to sell me the intake separately but after actually reading up on it i declined and used a k&n
replacement air filter in the air box. i noticed a bit more power when i changed, it just needs cleaned and re-oiled every x miles, but worked out better.
so just read up about intakes to see what others have done and what their experiences are. not sure about turbos in a rx8, it isn't quick compared
to an fd, maybe as quick as a fc turbo II, i don't mind n/a at all as i am not into speed, plus n/a engines last longer. though turbos would be fun i guess
ask ian he installed an elford turbo into one of his fb's and would be able to advise.