Ive been boosting my 2 alts trough instances aswell with my 70 mage.
Started in RFC, then to SFK. i stayed in SFK till level 20. (about 4 runs per level).
They are now level 35.
it SM Cath starts with about 7K xp per level, (unrested) and now at level 35 they get between 13K-14K xp rested per run.

As i do a full SM Cath run in 10-15 minutes, this is pretty nice.

I keep on doing this till they are level 47 as then my other chars will join the group and we will be doing ZF and stuff like that.

i would say boosting through instances is the fastest way to level. but you do miss out on training your multibox skill, but that is also trainable at higher levels.