Cyclemeter on the iPhone works great with BT low energy sensors. The main battery drain is from the display. I find with mine that I lose about 20% of the battery capacity per 45 minutes to an hour. It's just a few percent of the display is off. I have an iBattz battery case that I mount to my Quadlock stem mount and that, with the iPhone's battery and the battery in the sled is good for 8-10 hours (plus you can carry additional batteries if you want).

I think that if you used Cyclemter with the RFLKT display from Wahoo Fitness, the iPhone battery would last 5+ hours (at least).

J.