Well this exact question (probably same post) is on our local forum so I will cut and paste in my answer from there...
I hope i can be of assistance here. Experimented a fair amount on a low Y single flag bar, the north quad and also an older five line north bar on the 9 and 12 dice, as well as the 2011 dyno while I'm at it, but your question is about the dice.
The low Y bar as it should opens up the canopy. This is very noticeable especially when tethering and in flight as well. The high Y creates a much tighter C. On the Low Y, your low end is improved, Turning is faster, hangtime seem to slightly improve, and loops become far more direct and consistent. You lose some of the 'C' qualities and the 'stable' feel of the dice, but marginal.
But with everything there is a catch. The throw length before you stall the kite is reduced somewhat. Hence the plug and play, hooked/unhooked features of the dice on its own quad high Y bar gets a little more fastidious. In that you need to work with your trim a little more depending on wind strength and water conditions. This is far more noticeable on the smaller 9m than the 12 to a point where honestly you are better off on the north High Y on the 9m kite. You lose too much throw length before you stall it.
What is also apparent and this could very well be related to the above, the kite does tend to be more stable at its high end on the high Y north bar. So as with everything what giveth with one, taketh with the other. So...I use the low Y on my dice 12m for when I'm going old school on the flats and for freestyle, and the high Y when in the waves. The 9m just uses the quad, it performs better.
The dyno 16m I run the low Y 4 line in lighter winds (from 11 to 15), then 15 up its on the five line north bar. In terms of single line flagging on the dice, sorry I should have tested this, but I never have had to. There is absolutely no reason why is shouldn't work though. It does work on on the dyno though, but this is built to take a 4 line bar anyway and infact you shouldn't run it on their own 4 line high Y anyway.(the 2013 on on with no pulleys)..which is somewhat weird.
That all being said I rode yesterday the 9m on the lower Y switch bar in frontal stuff and it performed really well, but again we didnt get right up there with the winds. Definetly felt more at home sending powered loops though and that as on 23m lines not 22 on the quad.
Flagging not sure, someone should test this, but in 14 years I've only ever had to punch out once or twice...you watch it will happen the next time i go kiting now! Must admit although the kite stays powered a fair bit, I prefer the double line flag in the surf, keeps some power in the kite, gets it up quicker...but if shit hits the fan nothing beats the single line flag.
ps that all being said, if I use 23m lines on the low Y and 22m on the quad High Y how far is the actual Y from the kite...what is the difference really relative to back line length.