Technically, swapping the hard drive in any Check Point appliance with one we didn't sell would make your appliance not supported.
That said, it's entirely possible to do (either cloning or simply installing the OS on it).
As for disk performance, what software release are you on?
Because in R80.20+ if you do a fresh install (not in-place upgrade) we use the xfs filesystems, which should provide a performance improvement.
Not quite the same as putting in an an SSD, but it definitely helps.
Your license is tied to the Smart-1 appliance as well.
If you want to run in a VM, that requires a different license.
On the topic of SSDs in general, we do offer SSDs on some gateway appliances.
However, we don't support local management on these appliances specifically because the amount of disk writes for management can prematurely wear down the SSD drive.
If you are going the SSD route, I would make sure the SSD is rated for heavy usage to ensure you don't suffer premature data loss.