You can (at a real push) go for a 4 piston Porsche brake caliper setup (the NQSBBK) for around £600 but you'll be using calipers that'll probably show their miles, stock 312mm discs and normal everyday brake pads.
The 345mm OEM BBK (easier calling it that, it encompasses all the variations) is a well worth upgrade. My girlfreind drives a similar powered GTI to mine but runs stock brakes and you can tell the difference at any speed, both running OEM discs and pads. Another advantage with the 345mm OEM kit is that it fits behind all wheels with no spacers and even the space saver spare fits over them (not that you should ever use that on a front wheel!). Hope that helps you out.