Hi guys
Is it possible to exceed 300 hp with standard exhaust line in a 20vt? without catalyst

It is possible, but only just. The back-pressure will be very high, meaning that the turbo is having to work harder than it should. This in turn means the exhaust gas temperatures will be high

The first two or three feet of exhaust is the critical bit, so I would recommend you find a larger-bore downpipe as a minimum

okay. Thanks.

I understand you. I will put a downpipe of 3, and also the intermediate section of 3, and leave only the standard silent end, not to make much noise

You will probably need a centre silencer as well - the exhaust can be very "boomy" at cruising speeds with only a rear silencer