I've had my 7.8m for a couple of months but unfortunately only had a few sessions on the water.
My RSS 7.8 mk11 was my most used sail so I expected that jumping on the new one, with the weird looking back end, would be a seamless transition.....wrong. The sail feels different and rigs different. The clew felt a lot higher, it trimmed differently and my stance needed to change. So I can confirm that the changes to Mk111 haven't been just a cosmetic marketing upgrade.
Visually the rigging differences are noticeable, under the boom sail looks fuller and seems to have more twist up top but I'm still playing with the setup. The clew looks weird with the huge cutout. But it was interesting to note that the boom hasn't gotten shorter with the bigger cut out, the clew has grown in length past the boom end. 
. The luff has gone down from a 490 (Mk11) to 460 mast, so the whole thing is more squat.
The sail feels supple on the water but the rotation isn't as smooth as the Mk11. I've had the sail out in 5 to 20 knots, but rigged differently every time, so I cant say I've found the optimal set up yet and it needs to stretch in. All I can say is that no matter how it's rigged, it's fast 
. Using it in 20 knots, once settled in, it feels quite relaxed. I was always too lazy to change down when the wind came up so got used to hanging on to the old 7.8m in a decent blow, I'm not sure I'll need another smaller slalom sail for my local conditions now.....transition from 7.8 Slalom to B&J, easy. But a 6.2m would be nice for the 25-35knot days!
Just a word of caution when rolling up the sail, it seems prone to wrinkling just above the batten near the clew cut out, so careful handling may be required.
I'm really keen for some more TOW to get the sail optimally dialed, I can see my PB's being regularly updated on this rig. 
