i think this kind of idea is great; taking a large chunk of another trackday. i couldnt do the 2nd trackday at cadwell last year but did silverstone late in the year with quite a few other members and it was great to be all together in the same garage. lots of us do quite a few trackdays and if things like this are arranged early enough, i for one, would make an extra effort to go on these ones as opposed to just booking up ones on my own. well done to henners for trying to get something arranged.
I've never done the indy , but I have done the gp and found it a fantastic circuit. Took me a while to get my head round it, well worth it in the end. Even though the indy isonly half the distance of the gp, a good majority of the best bits are in the indy section.