Your title suggest a broad question, but your post is very specific about the 11 m2 Peak4?
Anyways, I úse 28 m2 with the Peak4 in 8 m2, otherwise too little power.
29 m on my onestrut LEI 10 m2. I have sold now as same lowend as above 8 m2 and takes longer to rig.
30 m on my high aspect 11 m2 racekite (sold now)
30 m on my strutless 12 m2.
30 m on my double skin UL 15 m2.
And when I had a Peak4 in 11 m2 I used 30 m lines.
All of above was used in 5 to 8 knots so I can answer yes to all, always using longer than 25 m for sub 9 knots.
When I used LEI's for foiling I used 26-27 m lines for everything also for the small ones, but this was in more than 9 knots.
For Peak4 in 6-5-4-3 m2 I use the std 21 m lines, longer does not help, but shorter I havent found any advantages either.
Peter