Short answer, no.
I can’t imagine the 2.5 inch exhaust being a restriction until you’re making close to 400whp. You would likely gain some power after a big turbo and going to a 3in but it wouldn’t be a substantial amount. A 2.5 inch exhaust should flow plenty for the power that you want to make.
This is the right answer. While it might not be considered the “ideal” size above 250-300hp, a 2.5” is typically good for 350-400hp without excessive restriction. Hence the reason a catback on this car is not really considered much of a power-adder. Even the stock 2.17” OD system is good for anything the stock turbo can output (essentially 250 crank on E30)
3” is optimal for the 350-400hp range to pick up a couple extra hp, but won’t be a restriction until you start hitting at least 500-600hp, well above where our engines are normally tuned. So unless you’re considering bonkers with a fully built motor with your big turbo, the 2.5” isn’t going to hold you back too much.
This is basically using info from Corky Bell, one of the noted pioneers of aftermarket turbocharging - the criteria is essentially that exhaust gas velocity not exceed 250 ft/s, and “ideal” for a N/A engine being about 190 ft/s. Turbocharged engines are also less sensitive to exhaust diameter tuning, as you primarily want lowest possible pressure at the turbo outlet, whereas N/A engines benefit from having some exhaust velocity/inertia to help evacuate the cylinders.
