I've no experience with petrol in Mk5's but, for what it's worth...
There is a guy on the Saab forum I frequent who works for Shell and Nitro+ is NOT supplied to anybody else and it is mixed at a specific refinery, not additives added at the gate as the tanker leaves the site.
On my turbo Saab I used to run Tesco Momentum 99 until I got caught and needed to fill up with no Tesco's nearby I went to a Shell garage. 50 miles after filling basicly empty tank I noticed a difference in mid range torque. Blind test because I did not know about all the additional lubricants in Nitro+ at that time.