Not exactly sure what the difference is with shorter lines, but you can definitely feel the difference. It's only a benefit when riding with power. I now ride about the same sizes I'd ride to be powered on a twintip (15m up to 16 kts, 11m up to low 20s).
The benefit that I feel from the shorter lines is you can get the kite where it needs to be in the window much quicker because it has a shorter distance to travel. If you're overpowered and need to move it up in the window and slow down, you can get it to the top of the window before you get pulled over the foil.
Also when the kite is surging in gusts, it doesn't move as far forward and back.
I wouldn't have guessed short lines make that much of a difference overpowered, but they definitely do.
When I get to my low end though, I'm looping the 18m and it isn't pulling me up on the board at all. About that same time, it can't complete the loop and just backstall onto the water. In that case, I feel like a longer loop wouldn't help because the kite just isn't generating enough pull to do anything, even in the middle of the loop. An 18m foil kite on 20m lines in 5 kts with bar pulled in actually takes pretty long to loop anyway, much longer than a smaller inflatable. Maybe I'd get an extra few tenths of a knot of low end from longer lines (I haven't done much testing) but I don't think it's worth it dealing with the longer lines. Foiling on the 18m is no fun on lines longer than 20m for me.