Hello and Welcome to our community @wgriffa1!
In the past highpoint has been fast enough to release driver updates. But I just checked their site and you are right. It says supported up to 4.3. Now, the same sources may very well build on 4.8.7 kernel. @bkelly from 45Drives may have tried that already. If he doesn't reply here, give them a call and ask.
We can also help you with building the module if you need. If drivers cannot be built on 4.8.7, you can continue to use Rockstor on 4.3. It's just that you'll be missing a lot of btrfs improvements that came along with recent version. If you need technical assistance with any of this, please feel free to e-mail email@example.com