I've never put spacers on a GT-R, but assuming they have the same stud length as other Skylines, 20mm will cover the stud whereas 15mm doesn't. Can you run an extra 5mm? Have you had a look at the wheel's mounting surface? Aftermarket wheels tend to have depressions in them, which wouldn't cause an issue with exposed studs.
66.1mm hub > nice, snug fit. You tend to see 73mm or so hub diameters for a wider vehicle compatibility. Pretty easy to get hub rings. GKTech ones are made for Nissan hubs (66.1mm).
If it really bothers you, you can always carefully cut the studs down (file the tips a bit so it centers the nuts when you put them on).