150 entry's
15 heats (10 cars per heat)
10 minutes per heat (3 warm up, 7 min quals)
150 minutes per round (2 hrs 30 mins)
3 rounds (7 hrs 30 mins)
Racing starts at 9:30 am and is over by 5pm
Looks like it should work to me, and thats adding an additional 30 entries. You should have room to squeeze the GT class in. Provinding a tight schedule is run and adhered to.