Hiya mate, it's not really worth going to the 259mm brakes, for the work you may as well do the 280mm conversion. Bear in mind if you do this it doesn't always fit behind all 15" alloys easily. It depends what you want really, but either way make sure the discs are vented and possibly drilled as well. Oh and if you do the 280 conversion don't forget the master cylinder upgrade as well.
Be carefull of competion pads, although they work really well, they don't always last to long! Guess that's why they're for competion's eh?
