I agree the standard brakes are very good , I haven't long had my 197 and i'm very impressed with the brakes , the previous owner fitted new pads which he told me were ' brembo ' pads , haven't checked to see exactly what they are but they do stop very well.
I did the cliosport trackday recently in Llandow and also took my 172 rally car so spent most of the day in that , so only went out for the last session in my 197 , I didn't give it full beans but the brakes coped very well
As most things there is always room for improvement , a larger 2 piece disc and bell will be lighter than a one piece disc so you have less unsprung weight so will give the suspension an easier time , as the bells are made of alloy they will dissipate heat quicker so should stay cooler , and transfer less heat into the hubs and bearings , and as the discs can move on the bells slightly when they are very hot as on a track , there is less chance of them warping and going out of shape.
By going up to 330mm discs you have more leverage from the calipers as they are spaced out from the hub.
There are no real downsides to fitting the larger 2 piece discs bar the cost of them , for outright performance they will be better than the standard one piece discs.
I am having a set of front and rear pads made for my car , i'll be fitting the new EBC Orangestuff compound and a set of 330mm 2 piece discs and bells as soon as the pads arrive.
I have bought my car to do some track days , I have already changed the tyres to 888's so it should handle and stop much better .