Then you'll be fine. yes having the 70 in group will knock down the xp per kill, but it will then be triple by the RAF bonus. People have been powerleveling lowbies with 70s in stockades and SM for years, the xp is still good even without RAF, since the mobs are elite and higher level than the lowbies and you get a group bonus. You'll also get more xp per kill with 1 70 and 4 lowbies compared to 1 70 and 2 lowbies I believe.

And quest turnins are also extremely lucrative for speed leveling even without a 70, for example: Howto: 1-60 RAF

People have also been very successful mixing quests and boosting by grabbing all the quests associated with that instance before getting boosted through.