This was my experience as well. It isn't really an Octopus thing (you could kill Octopus and still have the poor framerates). For some reason my machine kept tipping over at 5. I could run 4 without issue. Once I turned down some of the settings in-game I was able to add the 5th without issue.

Get your toons a port to Dalaran and see what your framerates are there. That will let you know if your rig is dialed in.

Good luck!