After rereading your post I think I've answered the wrong question. You didn't want to improve the REV's low wind performance, (SORRY) rather seeking an explanation on why the foil flew differently on your REV handles. My bad!
The difference is less drag on the thinner lines, as well as your tuning on the leaders from the REV handles. A foil (or stack of REVs for that matter) would have much more forward drive as you wouldn't normally be backing up or intending to stuff the sail with maximum pressure always. Bleeding some of that pressure off of the bottom/trailing edge is normal flight tuning. The Rev handles probably has more "DOWN" in the tuning, so you're capturing more pressure, that's quite necessary in low wind conditions.
Shorter handles are smoother in transition between forward/reverse + they work better when used with a 2-handed effect technique (one handle starts the action ~ the other stops or partially restricts it). Long throw handles aren't smooth, they are about maximizing the flyer's input thru an extended action. They will reach further, so you don't have to! They allow for dramatic changes, that equates to twitchy for some or as I like to call it responsive. You can dump all the pressure with a quick flick, then snap it back to loaded sail again just as instantly.
Thanks so much- actually you first reply has also been of great help to me too!
But I think I've worked out what the biggest difference is... I think when I put the rev handles on I took better care of my flying style as I didn't know what to expect. Insted of my usual foil flying style of " how much drag can I get from this thing" I started to see how much control I could get. Now I can get it to fly with the heavy handles and lines but far prefer the rev set up ( also makes for quick kite changes when the wind drops )
If there is one thing I have learnt about revs it's that people are so keen to help other flyers- and thats the kinda people this world needs!