Hey Jason.
On a normally aspirated car any increase in power from just an exhaust will be minuscule. we're talking 1-2 kW max. Turbos are where most of the exhaust benefits are found in terms of power.
The main point of these exhaust mods on the GT in my opinion are sound more than anything...
The 2.5" Borla system is a good improvement on the factory system both in flow and sound, but going to a 3" system will reduce the exhaust gas velocity (same gasses going through a bigger pipe will move slower). Why is this bad???
Extractors work to essentially suck the exhaust gasses out of the combustion chambers, and this works by using exhaust gas velocity to suck gasses out of the chamber to make room for clean air to get maximum power out of the combustion mixture.
Moving to a 3" system will (at low rpm) reduce the exhaust gas velocity thereby making the extractors less efficient at sucking out exhaust gasses, so you'll see a reduction of power from the engine at low rpm. I've seen this on all sorts of cars, however on turbo cars this is less of a problem, and on supercharged cars the benefits of 3" outweigh the losses you'll see at low rpm.
So my advice is to stick with 2.5" unless you're planning to go supercharged.
That said the losses of the GT may be unnoticeable on such a large engine, so the benefit is more around cost, as the 2.5" systems are usually cheaper.
Does that make sense? Let me know if I've not explained something sufficiently and I'll elaborate.