I did the 2.5" with stock cats, track exhaust (no crossover). Keep in mind the 2.5 vs 3 inch only refers to the coupling at the front of the Corsa exhaust - both the 2.5 and 3 inch systems are all 3 inches in diameter, just that the 2.5 system has a 2.5 inch coupling to mount up to the stock 2.5 inch cat coupling.
Crossover is quieter, which is nearly always an issue on a perf car, they have strict quiet regs they need to hit, also probably some torque impact at lower range, but sound is the main thing IMHO.
Main diff in stock vs aftermarket cats is in sound; stock cat with Corsa sounds louder, my wife thinks it's fine, and is much much cooler. 3 inch aftermarket cats, maybe another 5hp, much more sound, some say too loud, some say perfect, and should be even a bit cooler still. If we had rear exit exhaust I'd do the aftermarket cats, but with the tips like 30 inches from my ear I didn't want it too loud.