Nice to hear others also do these designs. And yup, would need the magma tile 1 tile further out if using highest speed roller....Interestingly, that system also has 24 roundtrip time. Hard to say which generates more mist, but it does have the advantage that for this EW rollers + corner tiles DO work, allowing player to use 10 long rollers instead of 1 long ones, saving labor and material cost.
(Unfortunately, seems attempting to lengthen further to make center wagon-passable via having the roller on track-ramp shotguns out the water - besides, wouldn't be able to turn off power on that one.)
If I read your diagram right, Ws are walls there, correct? In which case the magma is produced not by skipping but by cart falling in. Would work fine with iron carts but unviable for wood cart as in the OP (maybe useful for those volcano embarks with 1st summer siege).
If you do have iron carts, tbh the simple minimal spiral to raise cart out of channel and drop back in might maybe produce more mist? Longer time between mist spawns - however, longer falls seem to have better odds to make large mist plumes. With testing seems 1z fall gets about 0 or 1 tile spread 50-50 in six attempts, while 15z fall is 50% 0 spread 50% 3-4 tile spread.
Yeah, I also noticed mere skipping doesn't produce happy thoughts, which is why I abandoned the idea for a long time, using track stops instead - though I only used 2 submerged ramps to make 4x1 line or circle.
Speaking of which, skips/magma-efficiency wise the OP design is only 25-37,5% more efficient than as the old impulse-powered skipper. The real savings are in space, but..
Hm. Seems I once got ▲▲_++_▲▲ to work and generate skipping mist with both 5-7 water and 5-6 magma (didn't test 7) but can't replicate it, using same weight cart. Maybe got lucky with subtile coords, but that'd make priming/stopping an issue anyway.