A quick update on this. I bought the REV D as I was told by a dealer that my REV G might be leaking.
Unfortunately he lied and VW wouldn't refund my money so I'm stuck with an extra DV.
I have tried both the REV D and REV G but I have to say on a stock GTI I'm not noticing much of a difference. If I had the option I'd probably go for a REV G.
For those that don't know, D is a piston and a mechanical top and G is a rubber diaphragm that will seal better but can rip.
Here's my new REV D