I have set the test cluster jvm heap to 30.5g. I can see it is now using zero based compressed oops.
Heap address: 0x000000008d000000, size: 30512 MB, Compressed Oops mode: Zero based, Oop shift amount: 3
Based on a few initial tests using long running scrolls. I can see still heap spikes occurring but the highest I have seen so far is 26g.