you can go to about 210 - 220 on a ko3 turbo, upgrade to the t4 and you'll need to increase injector size and remap ecu to suit. the cat is very limiting. you'll obviously need to replace the exhaust manifold with the t4.
about 260 - 280 bhp is probably limit on std internals, so i guess if TT/s3 injectors fit they might be upto the job.
i think you might get some lag on a t4 though, as i think they are quite big units and no roller bearing to allow faster spool up. most people are putting on gt2871 or gt3071 series turbos.
change the internals and 600bhp has been seen on these engines, reliably i'd say 400bhp is to be expected.
dual mass flywheel i think needs changing if going higher bhp, convert over to a single mass item with an uprated clutch. not heard of gearbox issues though mine is making some very interesting noises since boosting up the power.