You can go much further than that Dom.
You can run 9" et 41 quite happily with 215/35 tyres , every Bentley wheel is run like this. Often with little to no rubbing.
I wouldn't go higher (having done it...) than that, or equivalent around 8.5 et 40 front with a 205/40 or a 215/35 tyre.
At the rear you can go much wider and lower offset, plus you also have factory adjustable camber up to 4 degrees.
You can run a 9.5 et 35 with a 225/40 quite happily without rubbing at all.
9.5 et45 might catch on the inside no?
Why do you want wider rear wheels anyway.. It will look crap. On an m3 or a 911 fair enough but this is a golf!
8j et50 on mine and i think thats about the perfect fitment on a golf.
Might look crap to you, but to other people the wider rear looks much more purposeful.
I ran et 56 front et 36 rear on a 7.5 for 2 years on my old TDI and it looked far better and handled very well, if anything it was more stable on the back end due to the wider track.
9.5 et 45 is fine on the rear.