Meh, the gap for cooliing's pretty immaterial, given that you've got wheels with gaps flying around all over the place. So long as a fag paper will go between the caliper and the rim it'll be fine - a couple of generations back the M5 actually had wheels that worked like fans to push air past the brakes. Always though that was particularly neat.
What is the smallest wheel size/offset that will fit the dated Golf5?
As a guide on weight, the Team Dynamics 15"x7" rims I run weigh 6.6kg each.