1. I had a Venomous X in mine, its 160mm high, the V6GT is 165mm high, I had no issues.
2. Depends how many cables you have and how you can arrange them. I don't have a modular psu, although it would be easier if I did but the case handles my many cabled Corsait TX750 ok. Yes it takes some time to arrange them properly and it has cable tie brackets that help but plenty of room. I have found if you slide the HDD cable up and under the HDD chassis it works a bit better.
3. Think I just answerd that.
I'm very impressed with the 500R, having now installed a H100 cooler and messed around with the fan set up its cooler than ever.
I'm thinking of upgrading the 2 front intake fans as they don't shift a huge amount of air but I am talking about a seriously OC'ed setup.
Hope this helps.