The MK5 is designed to use the higher octane fuel. I won't go into it but its to do with how the FSI engine works (if your interested have a quick Google of how the FSI engine works). Even a standard MK5 will see benefits from it.
Having said that its bloody expensive, I personally just use the usual stuff purely for cost reasons. Used to use it when fuel was cheaper and I didn't have mortgage to pay and tbh, I really struggled to see that much difference. Still, the facts speak for themselves.