I could talk for hours about my track day experience (at Road America), but suffice it to say, I had a blast with my BRZ. You're exactly right to say the RWD and overall lightness (2,700 lbs) is great fun, even if its not at super high speeds. While it is underpowered, I found that I could stay with cars 2-3 times more expensive in the curves. It was just disheartening to outwork the 'Vettes and Mustangs in the corners, only to have to let them blow by me in the straights. That said, it was slightly entertaining to see my car make it through the entire day with no issues, while many others retired early as their brakes melted or worse. If I can make any recommendation to anybody thinking about doing a track day, its this: get race grade brake pads and high temp brake fluid.
Anyways, not sure I'll do a track day with the GTI, but who knows - I may get that itch again and have to!