Until recently, the maximum hosts per HA cluster was 10. It's completely feasible to continue leveraging this approximate number, even though the theoretical maximum is now 32. There's no reason you need more than a certain number of hosts in a single cluster, even if you have far more hosts than that. This would keep the "volumes per cluster" around the 150 you currently have.
For your scenario, I would simply build your six new hosts in a new cluster - with new LUNs.
Are you building more LUNs than you need? 150 across nine hosts does sound like a lot.
