I guess I will stick my neck out and be the only to say that my 2.25" has been working great with my HR-TT up to 470 WHp. For sure it has seen over 500WHp during development caused by boost creep before my internal wastegates were changed.

I've never noticed slow spool-up and EGT has never gone over 1300 at the manifold, but then again, how long do you plan to be in boost? At steady idle, the EGT will be around 750.

I've rode in a GTM VHR-TT with 3" pipes and I can say it was too loud for my taste, but I'm sure others will feel otherwise.

IMO, if max boost at any given pressure is what you are looking for, sure go to 3" ... but here in CA ... I can assure you that the CHP will also notice it.;)

BTW ... any existing CBE for a NA Z will work with the GTM TT kit, so you may as well try it out before you decide to go bigger.