The real differences between the Speed vs a tube / surfboard in light wind:
1) Heavier riders can get going with the Speed.
2) You can use a fast twin-tip instead of a big slow floaty board.
3) Upwind is significantly better.
4) Tricks/jumps happen sooner.
5) The kite stays in the air better (especially the lightweight Silverarrow / Deluxe editions). This is also a benefit for lighter
6) You don't have to work the f*ck out of the kite the whole time.
(Some guys say they don't care about any of these things, and would rather stick with LEI's despite the disadvantages - don't bother trying to convert them, but don't feel bad when they get left behind either.)
The main disadvantage of the Speed2 is turning, compared to smaller, less powerful kites (duh) - but for its size/power, it turns quite well. The other disadvantage is that the Speed "only" adds a few more mph to the bottom end, so at spots where the wind is rarely that light, or never settles in that range, you can get by with a smaller kite if you're willing to skip a few days or wait around for things to pick up.
As for relaunch, IMO you don't want to relaunch ANY kite in light wind - as soon as it gets wet, it gets heavier... The advantage of foils is that they STAY in the air.
The questions to ask are:
1) How often are you in that "wish we had a few more knots" windrange? At some spots, it won't be worth $2500 just for a few extra days, but at other spots it might double your time on the water - and not just cruising time, it will also increase your jumping/fun time.
2) When the wind is light, do you want to park the kite, go upwind, and do tricks, or would you rather walk back upwind after a slow cruise while working the f*ck out of the kite? (I'm not judging your decision, just don't expect me to sympathize over beers afterward - I mean, I'll glady go for beer, but won't be able to hide my grin...