We run JVMs - running complex webapps on Tomcat - in production with 28 GB heaps. We don't have OOMs or chronic GC pauses. We don't reboot in between fortnightly releases.
We aren't running machines with 100 GB heaps, but i can tell you from personal experience that it's possible to exceed 8 GB without difficulty.
I doubt you'd have to look far to find people who were running with substantially bigger heaps on stock Sunacle/OpenJDK JVMs.