Chris, the 4 pots have their limitations, but you can work within them. The standard discs are really only pushed beyond their limits with track work or very fast road use. They're not vetned, drilled, grooved, nothing, just solid, so their big problem is heat dissipation, which obviously spreads to the lines, and subsequently the fluid in them, and of course to the pads themselves.
By fitting better hoses, filling them with better fluid, and by fitting better pads you can have a setup which works pretty well for average and fast road use, with a view to installing better discs when the standard ones wear out.
I've managed to get 33K out of my standard front discs but I'm on my third set of pads. In that time I've done 5 trackdays. Some people will think that I've not been using my brakes, but I'm quite efficient on them, and I like to go round the corners faster []
So depending on what life you have left on the front discs you could possible get in another set of pads or 2 before they'll need replaced, saving yourself a bit of money in the process.
If you want to really upgrade then the first option would be to move to the Godspeed kit. This retains the calipers, but spaces them out to fit thicker pads, and moves them further out from the centre to fit larger discs. This is supposed to be a very good kit and comes in about the £650 mark I think. That includes the pads and discs and all the gubbins required to do the work. Not braided lines though I think. Downsides - need to get the discs from godspeed in the future as they're made to measure AFAIK. Upsides - no need for larger wheels etc, and relatively cost effective.
Next option - Brembos off an STI. Cost depends on if they're new or used and varies wildly. Upsides - direct fit, plenty of options in terms of discs and pads etc. Downsides - need to fit bigger wheels as the WRX ones wont clear them
Option 3 - full replacement kit from AP or Wilwood etc. - COSTLY but GREAT!
Personally, I think I'm going to stick with the stock setup but get better discs next time round. I know the godspeed kit would give brembos a decent run for their money, but ultimately I'd just want APs, so the cheapest option will suffice.
MAYBE!