i'd only buy 20" if you have the calipers to fill the gaps, if your on stock calipers i wouldn't do it, would look like a riced out corsa. even brembos struggle to fill 20" wheels.
I have 20" on my XKR and i have giat 6 pot calipers with 355mm discs and big 4 pots at the rear with 355mm discs. and even i wonder if i should try and squeeze bigger discs in.