I made up a 55cm bar with 15m lines that I use on my 12, 9 and 7m Hyperlinks. As you would expect the kites become way more responsive and agile. Loss of power with shorter lines seemed nominal. Powered loops with the 7m were easy enough
I tried the 9m hyperlink on 20m, 23m and even 27m lines when I owned it. It turns pretty fast for a foil with pivotal loops.
If you are riding powered and know how to get the most efficiency out of the kite drop to 15m lines ( I use 15m on my 8m Soul), otherwise I dont think theres any dramas using 20m lines.