Yes, front and rear has same calipers as the Focus, it's the carriers and discs that are different.
Generally you want the more powerful on the front so it could throw the balance off.
Remember, bigger discs increases the unsprung weight, particularly the front 300mm discs on mine are pretty heavy. As above, better pads would be a good way to go but its your call.
4 pot front calipers would be a good improvement but more cost and work involved
2005 Fiesta ST
Milltek non-res, Newman fast road cams, Fusion Fabs custom inlet manifold