The Multistar mod where you open out the door opening a bit and move the cable outside sets your FC40 free.
Average of 13-14 minutes flying time on each battery makes the thing worth flying and takes some of the stress of constantly looking for the red lamp away.
Well worth doing, just look at the 4000MAH multistar mods on here, everyone says its the best thing they did.
Think of it as a correction of a design fault, 2100 was never going to get far, hence why they changed batteries.