Why partition it at all? Any decent imaging software will only image what's there, not the entire disc. Only time I would split an OS disc is if I planned to dual-boot with something else - and even then only if I didn't have a spare HDD knocking about.
Max score of a mechanical HDD is 5.9. SCSI/SAS/RAID may get more points but dunno. In any "normal" desktop you would need an SSD to get more than 5.9.
Exactly.
Sometimes it just isn't worth the bother installing software all over the place. I used to make that mistake years ago - small OS partition then struggle down the line when it fills up.
If you are worried about data [putting aside it should be backed up anyway] get a second HDD dedicated to storage.