In my opinion its going to depend on what transmission you have. Unless you swap that as well, as auto NA won't like BPU power - 400bhp ish. im manual and im going through the stages of NA-T shopping list and to be fair, it probably would have cost the same amount of money to swap to a TT engine. if you want 320bhp go NA-T, if you want 400 you'd probably be best to swap and it will end up costing the same amount of money.
You need to give yourself a power goal, this will depend on what transmission you have though. some people have had higher power on stock NA transmission but this is what i found recommending power levels:
http://mkiv.supras.org.nz/techo.htm
Go to the section automatic tt gearbox spec
W58 5 speed manual in the n/a is good for 260 rwkw,
Auto in the n/a 200 rwkw
Gertrag 6 speed 600 rwkw
Auto in a tt 350 rwkw
1hp = 0.735kw so..... 1kw = 1.341hp (ish) so we convert and we get:
w58 348hp
na auto 270hp
gettrag 804hp
auto tt 470hp
Ive seen LOADS of cars push these figures and have NO issues but its just an idea of how far your pushing your box. These aren't figures set in stone just some info I found on another forum. Thought it may give you idea.