I currently have the P zero also on the 19", they seem better on the GTI than they did on my Fiesta ST, however I will be swapping them when the time comes. They're quite well know to give a harder ride and not necessarily the performance to go with it!
If you are a member of Costco (worth looking into) they do great deals of tyres, at the moment you could get an UHP tyre Eagle F1 Supersport, (Michelin Pilot Sport 4S equivalent) for about £150 a corner. That's a good 50 quid cheaper than the Pilot Sport 4S that I've found on the likes of Black Circles.
The 18" would be cheaper still at around £100 a corner from Costco for a Premium or UHP.
I was tempted to do the same with the wheels but I really like the look of the Bescia's and intend to look after them. Also finding premium or UHP tyres for around 150 instead of 200 helped!