20m lines are the go for kiting in the waves, very fast and direct response to redirecting the kite this means full on power turns and gives you confidence to take on the lip. Longer lines i.e. 5m extensions for lighter wind days and or flat water boosting as you will get more pop and boost from 25m lines... I don't use Nth kites but assume you will get same diff in line length changes as my delta kites.... If you can get 20m + 5m extensions then you can try the diff line lengths and see for yourself without being locked into 25m lines...

yes shorter lines will extend the range of your 9 and your 12m too... I can kite in up to 25knots on my 12m with a 6' surfboard strapped on 20m lines but love my 9m in 22+ as the turning speed and lightness of the kite is amazing...

I have the 2010 Rebels and set up a 20 m Lines set from North( was used to kiting surfboard last 5 years on 23m lines) hated 20m lines on 2010 Rebels, killed a great kite, took me 6 -8 kites sessions to get used to 25m lines on Rebel and now love this length, kites are quick, amazing up wind and easy to fly. I kite strapped and powered so thought I needed shorter lines, not true. Love my 6m, 7m, 9m & 12m all on 25m lines.

PS. I'm 95kg and use a kite size smaller than last year on the new Rebels, kited last night in 15 to 20 knots fully powered on the 9m, last year I would have been on my 12m