Using a higher level toon to powerlevel up groups while receiving the RAF bonus is not SUPPOSED to work, but it HAS worked in the past. I actually used a 70 mage to powerlevel one of each class to level 60+, but that's right after RAF was released and before they updated their rules to include the statement that it wasn't designed to work that way. I haven't powerleveled this way since then (haven't had too... I was already done, to be honest). Too bad I didn't work on more alts at the time, oh well.

You may find that it works right now due to some fluke or bug, but it's not what Blizzard intends to allow. Without any official statement of whether they consider this an outright exploit, you are best to assume it is unless you find something to suggest otherwise.

EDIT: My mage was always grouped with the lower level RAF'd characters. This should no longer work.