The mk5 engine is stronger to allow for the increase in power.
The TSI engine is as strong as the ed30/35/s3 engine. 400bhp no problem. It does have oil starvation issues on track though.
The new s3 is reported to have thenewer TSI engine with a ko4.
The TSI engine from what i understand is actually stronger than the TFSI, and with a k04 turbo bolted on its makes a fair amount more power and torque. But as said above, it suffers oil starvation APR have worked out that this only occurs during cornering when G Forces go above 1G so basically you would need a heavily prepared track car running top end semi slicks on a dry day for this to happen. Chatting with APR, they have developed a fix for this which stops the oil starvation so it allows them to keep on racing their Mk6 GTI race cars so its no longer an issue.
Back to the OP.
Id keep the 2.0TFSI quattro ove the GTI all day long.
