That guy was full of shit. Right off the bat, no one has ever even MADE an aftermarket head for a 7M AFAIK. You'd have definitely known if it was cammed, noticeable lope at idle. Intakes don't do a damn thing for a stock turbo, the factory airbox flows as well as most aftermarket setups. Fastest stock car I'm aware of ran a 14.5. Mid 14 to Mid 15 is average, most of those motors are getting pretty beat and they had head gasket issues from the factory.
I've had a BPU+ 88 and an 89 Targa (both turbo cars) with just a catback... 88 could have hit 11s if it hooked and didn't blow up every five minutes (redneck rigging, bought it like that. Fast as balls, real fun, broke every time I drove it lol). 89 would have easily destroyed my 96 GT (auto vert turd) or my buddy's mint 98GT (hardtop 5spd stock 80k)... Most of those cars are driven old-woman style or they break often. Parts are expensive, assuming you can find them, and they're a PITA to repair. However, bolt ons in a MkIII (86.5-92) usually get you 300-320rwhp with 350+ftlbs. If it's a hard top, it'll be decently quick, if it's a targa, it'll still be a mid to high 13s turd. Bolt ons in a MkIV (93-98, jellybean) usually net a little over 400/400. Mid 12s if well driven (rare).
I'd rather go slow, look and sound AMAZING, get decent gas mileage and be able to drive every day than look at my Ford Probe-esque 4000lb paper weight. So I came to the dark side
Btw, there are stock 2JZ-GE-Ts (factory non-turbo motors with turbos bolted on, poorer heads cams different pistons etc) making over 800... Believe the GTE record is around 1200rwhp right now. Last fall a guy named X-Man set the new 7MGTE (87-92 turbo motor) record at ~700rwhp, only mod being a metal head gasket with ARP head studs. That motor was rebuilt a few years ago and has seen at least 50 track passes and numerous street outings since then.