To correct the camber I still reckon that you would need some non-Seat bits. Lots of people are probably just living with the increased inner edge front tyre wear as sorting it out might get a bit more expensive if your annual mileage is quite low.
All the Cupra or Powerflex TCA rear bushes will do is to stiffen up (tighten up the TCA movement) the front suspension assemblies, which is good - but they will not change the camber at all, though they will reduce the tendancy to suffer from power-on increased toe-in which will also wear the front tyres more than is usual.
I was lead to belive that the CUPRA wishbones were slightly shorter than the other Ibiza's because it is lower out of the factory.
Camber can be corrected by having the wishbone holes slotted where the outer ball joint bolts to the wishbone - then bringing the ball joint in towards the car effectively shortening the wishbone.
0.5º negative is the recommended setting for a road car to get good tyre life.
I get about 18,000 from a pair of front uniroyal rainsports without rotating wheels or tyres to try and even wear out.