Depends on the 15s really.
Generally speaking if you're going up to 15s you need to find a set which have a lip, this makes the spokes look smaller and stops the car looking so over-wheeled.
But the issue is it either looks awesome or completely awful, there's no middle ground. I think my old 15s looked great, but almost every other Mk3 on the same sized wheels looks a bit wrong on them.
285mm brakes are overkill on a Mk3 as well, unless you've got silly power. The Audi or Vauxhall 256mm items are more than up to the job, and they both fit under 14s. And 15s won't give you much, if any, more traction. But they do give a much wider choice of tyres, including my personal favourite the Goodyear Eagle F1.
TBH though, your existing wheels look huge for 14s (spokes that go right out to the lip tend to do that).