I had 2.2(ish), 6,3 and 10,5 m C-quads. Used them mostly for buggy/skiing.
Had those inflateble tubes for them too, but didn't see much improvement in waterstarting
After these I got myself a Mosquito 7,5 m, brand forgotten, that had condoms(!) as air-valves and hence closed cells. A few cells had inflated bags in then too, but that mostly made the kite heavier.
Is the C-quads still used in buggy-racing?