From my research, it would appear that the XP penalty wouldn't be worth it unless the 70 was on a seperate account.

If the party is lowbie+lowbie+70 then the XP penalty will still be bad, but not self-defeating. If it's just lowbie+70 then from what I've seen it's no faster than normal levelling.

This is assuming you're not using the pally/mage tricks in instances to get a single lowbie full XP for kills.