The view am getting is for the main.
MK1 Landy mainsail is highly efficient sail over most wind conditions which has to well control to get the best out of it.
GP
Have more up top so are easier/simpler to use in low wind.
Plus are more easier to dump wind in high wind.
Downside more sheet load, and most likely less efficient in mid strength winds.
Kite
GP prefer to be sail higher.
I'd not quite agree with that.
Manisail Landy mainsail is a good sail, but the GP IS Better all round. But, because the GP sail is flatter, you do have to be more accurate to get the best from it.
Kite It does need to be driven harder, so you head up more to start with, but as the apparent wind builds, you sail deeper. When sailing against other Stealths with Landy kites, I sail the same (or deeper) angles, but I am sheeted in harder as the apparent wind as built more and thus I am going faster.
It's better all round!
You also need to remember that the GP sails are at least two evolutions ahead of the Landy sails now (I am not aware of any developments on the landy sails since 2007).
