Agreed, leave the budget tyres well alone, the handling won't be the same and you'll only have to replace it again that much sooner as cheap tyres don't last very long. If you shop around then you'll find that you can pick up another one of the Pirellis fairly cheaply, it is a big tyre at the end of the day, so it will cost a few quid. Try somewhere like
www.mytyres.co.uk or
www.blackcircles.com for options.
Apparently the version of the P-Zero which we've got was actually "developed" for the ST, although how much of that is true I'm not sure.