theres a company in america ive been in touch with (will dig out the name)
they've done a couple of N/a project builds and there able to squese and extra 30-40 bhp with not to much done and not too much expence either (ie no machine work).think 1 of there n/as is running 275 and the other 267 or sumthing (dyno'd as well,not jst flim flam)...but after that your looking at heavy machine work ie. skiming porting and heavy cams etc (all of witch are not cheap and would prob be about the same as a Turbo conversion anyway), at least that what my reaserch has proven.
to be fair an extra 30-40 bhp is more than gonna make a differance..hell it will make a huge differance but only do it if you looking for a bit more poke and arnt too fused about it...if you want a beast of a supra a Turbos is the only way!
p.s i personally would only do a few basic things and wait till i want a turbo or get a v8 in it
(waits to hear the abuse of turbo fans)