I agree that we are in unknown territory...
There are actually a slew of things to consider about getting a full 3" turbo back on an unknown platform but here are a couple things related to OP.
1: In my experience, it's not hard at all to get over 20HP without upgrading anything in the exhaust at all as long as you're tuning a forced induction car. Depends on the car though.
2: Depending on the size of the wastegate on the OEM turbo, a full turboback system can introduce boost creep problems where the ECU is unable to bypass enough exhaust through the wastegate to keep the turbo from continuing to build more and more boost out of control as the RPMs rise. More flow is great as long as you can maintain control of the boost.
Frankly, not enough stuff done to these 2.5T engines for us to know these answers yet