I am about to buy 2 Glk Lift. First I thought I go for the 18 and 12 but after I red some reviews I am not really sure anymore. I weight 83 kg and want to kite from 5 m/s. It seams that the new Glk has a lot more power than other kites the same size so perhaps the 15 and 9 is the right choice?

Arnold

I would choose 18+12, because you'll have a huge windrange

12 + 18 is the right size for you.
I have the 12 now and it works for me up to 28kt gusts. I am 75kg.
The new GK have more range than previous kites. You will not regret buying them at all. They inspire confidence in my riding. Confidence is not quantifiable in terms of aspect ratio or other numbers but in terms of predictability and feel.
I will get an 18 as soon as I sell my old kite!