Andrew, if you are coming up to Bali anyway and want a day's popping, then do it. Few folks understand just how many 30 kg plus (and plenty 40-50) GTs are caught in Bali and Lombok waters. It can be really hard work and the sites are not ones which produce boat-loads of fish. But the quality is there.
The only issue is the time of year. August-September is not great. Leave it as late into September as you can.
The key Lombok sites are reachable on a long day trip from Bali and give you a better chance of catching GTs. On the other hand, the main Bali site has produced bigger GTs, if less of them.