With regard to line lengths, I have had a variety of experiences over the past few weeks with fall winds kicking in. I've only been alternating between 20m and 17m lines. I would say the difference is the equivalent of 1 knot ... to a little less than 2 knots in wind strength. I only use the 6m with 20m lines. I only use the 3m with 17m lines. So the line length variations are only relevant to the 5m and 4m kites. If it's 12 - 14 knots I will use 20m lines on my 5m, above 14 knots I will use the 17m lines. Using the 17m lines will make it comfortable to use the 5m in winds up to 16/17 knots, but I might consider using the 4m with 20 lines from 14-15 knots. At 16 knots up I will definitely use the 4m with 17m lines.
In practice, I often find myself on the "wrong lines" because the wind picks up, or drops during a session. I've never come in to "change line lengths" - it's not worth it as there's no guarantee the wind won't pick up or drop again. I've become used to riding much more powered up than I used to with the Peaks. It's nice to be very lightly powered in good swell/waves ... but equally, it's nice to be powered up and be able to ride fast & hard upwind. I've had a number of sessions ridiculously overpowered with the 3m (30-35 knots). Not much fun really, but still doable because sheeting out the Peaks (lots of flapping) allows you to manage even really overpowered.
Shorter lines - mine are 14m - I only use in "real" waves. You lose a lot of ability to generate power, but gain in immediate responsiveness - I guess closer to the feel of winging.