Indeed SWAP ideally must be on separate drive then Moodle, that is why I mentioned in firs place keeping O/S + SWAP on one drive and rest Moodle + moodledata separately, this is as well recommended by Moodle, I haven't though used AWS myself, but if you are getting your services provisioned on SSD Drives, then keeping them together wont be too much of burden on the system.
As what I assumed you are setting up your own hardware, in that case I would have suggested the above solution. Also have you calculated the costs? As still 94% of the VPS's are running on standard hardware architecture, down from 98% in 2009, still the shift towards cloud is very very slow.
Lets hope someone with AWS experience would come up sharing their experiences.