if you own an NA and your going TT your going to hit 2 problems if u want massive upgrades. first being gearbox limitations, second being TT engine will hit 400bhp and without costly twins or going single you won't see much more. going single on a TT ££££££££££££
the cost to put a TT lump in, then go single to get 500+bhp will be more than going NA-T, lowering compression and required modifications to hit 500bhp.
the only benifit in my eyes is going to be the reliability, but then NA-T's have been pushed to higher levels with no issues when done correctly. right set up, right tune, right modifications, no problem. going 500+ it wont matter what engine you use, as your want the best tuners around.
this is of course with a shiney new R154 or 6speed as your w58 will wet itself at those levels