Ask your questions.
Fuel economy will depend on your mods/driving style. It can be between as good as the NA to 350k's/tank. All depends on how much right foot you use. Generally it will be almost as good as the NA.
Compared to the WRX I would say about the same, I know my friend Doreen has a WRX and her fuel economy can be a bit terrible at times, especially when she gasses it, much the same with the Skyline.
I think the best bet would to be to find a car you are interested in and asking the owner is they will let you test the car for a week or so, most R33 owners are easy going people and I'm sure most would agree.