My father used to test tyres for the police, and has a lot of experience in the matter - including finding out what happens when you maintain 100mph+ on a run-flat that has already blown... But thats a different story.
Although not on a Golf, I changed from Continental 215/40/17 to Goodyear 225/40/17 on my current car and it made a world of difference. The Conti's were not worn, I just hated them, they were noisey and didn't grip that well.
Wider, low profile tyres will always be more prone to aqua planing, another few mm is going to make a negligible difference. All this talk of tyre wall flex due to more rubber on the rim is not an issue if you adjsut your pressure accordingly as TT eluded too. Especially for track use. And the extra tyre wall is just more rubber when cornering hard...