Can I suggest you use garage cars so that tuning doesn't play a role. You speak of using FGT's which can have a huge advantage if tuned right. Use like the Zonda R, Nascar, SLS etc from the garage. It would still be a good mix of cars
Also try getting in contact with as many GTP champions as you can get hold of to promote it.
Plus, I know you were being sarcastic when you said I like Tsukuba, but it is the worst track known to man. Use a wider track with bigger braking points. More opportunity for overtaking. Like using the small version of the Nurburgring track that the DTM's use.
And to top it all off your group idea is stupid. Isn't it just sensible to make 4 groups, 4 drivers in each and the top 2 from each group go through?