Not adding anything to the discussion here but for the record, I've been running my team through instances with a 70 rogue since level 8. No tricks, just group up, clear, reset and redo.

I did RFC from 8 until 10 and SFK from 10-18. I consistently got two levels per three runs through to about level 16 but it seems to be slowing up now.

As I've now done the SFK quests and I've farmed all the gear I can, I was going to move to RFK for a couple of levels before hitting SM but as I'm only two SFK runs off 20, I'll stay where I am because the time it takes to get from SFK to RFK and back is about as long as it would take me to run the instances.