You can get serious brakes for the LGTs, but you need to get larger wheels to fit them into. Brembos and such do not fit in the stock wheels, but there are bbks that fit if you get at least 18s.
There are some pretty decent coilover options in the more expensive range (teins, etc) or you can get cheaper coilovers for that track car handling with the track car ride. There are also all manner of little bits like roll center kits and bushing replacements. There are sway upgrades as well. Pretty decent options.
The stock 5MT is not a strong transmission. It can reasonably hold 275 to 300 awhp and that's about it. Upgraded clutch and flywheel will help that a fair bit, but if you really want the good tranny, you need to do a 6MT swap from a later year.
The EJ255s are generally considered to be a pretty stout motor. You can get that sucker over 500 with forged internals and the proper supporting mods. Stock internals can handle 300+ (at the wheels), but probably not a lot more than 350.
Spec.Bs are nice. I forget the list of upgrades, but you would probably be swapping most of them out. Get a GT and figure out which Spec.B parts you need if you need them (aluminum lower control arms instead of steel, etc). Don't pay the premium and waste your time on the hunt for a spec.b for a track car.
In the end, a WRX would probably be a better choice.
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