I agree that humidity will make a difference, I would say above 70% will slow anything moving through the air down.
On your other question, when I decided to get into lightwind I made that exact same decision, Mastair 12 or Warrior 16?
I ended up getting the Mastair which has the same low end range as the bigger kite but a higher top end. The kite is more technical to learn on if you've never flown a foil but worth it.
Apart from the smaller, high-AR foil having the same low end as the bigger easier one I thought OK so what am I gonna do if the wind picks up to 20 with a 16m foil in me hands, pray? Also I wanted to use long (40m) lines to get more window and low end and if a kite is sold with short lines that makes me think, why? Could be because it's slow, and maybe 40m lines would bugger its turning? Never flown a warrior so I don't know and many people love 'em, but the choice was also a new warrior or exhibition MA, no match.
Only prob is Flysurfer ain't making Mastairs anymore, they've gone psycho which should be great in more wind and like the warrior in light wind, so the mastair still rules there.
You'll probably find second hand MAs around when people can go psycho in a while.
Like Johnny sez you do need to work on the technique too, lightwind is all about apparent wind, it's really easy to heel the board too hard especially if you're used to kiting in stronger stuff, and you have to keep the speed up always, starting using the kite to lift you out of the water is good and dive the bugger right down through the centre of the power zone to get going, in the lulls point the board more downwind, also when the kite is going back up, go upwind on the down stroke or when locked in, you can lock these kites in pretty soon. I am just starting to get the hang of the MA, last evening was the first time I tried it on the longer lines and I was expecting slow turning and maybe other probs but it was sweet, seemed to fly even better, stable, fast turning as long as I kept it moving and sooo much power in that big window even in 7-9 knots, there were times I was even overpowered, blasting over the water, not a ripple in sight let alone a whitcap, makes ya lightheaded, try it and see.
I don't agree that lightwind kiting is boring, kitsurfing in 10-16 knots is a lot easier, in lightwind it's challenging and I love challenges.
You will be the fastest thing on the water without an engine.
Try it and see.