First, do you think you really go faster with a kite in winds above 20 knots, or do you just sail lower and as a result get to the mark sooner? it seems to me that there is a certain amount of increased drag with a kite, and it's not going to help your top speed in overpowered conditons.
How does a kited boat handle jumping over the backs of swells on a hairy downwind leg. i seem to remember reading some stories about jumping over the back of a wave and stuffing into the next wave since you can't head up much with a spi. in order to angle across the back of the next wave.
also a thought that may help explain increased stability fron a spi.: maybe the fact that the boat is being pulled from the front rather than from the main beam as you are with just the main helps stabilize the boat.
1 other question, how long does a spi. last if used in high winds regularly, and capsized from time to time?
thanks in advance.

in large waves you're going to want to surf with the spi up.

head up going up the back of a wave, then bear off as you're coming down. It really does work once you practice it enough.

Not always possible, but that is the best way to do it in ideal conditions. If it's honking at 20+ and you've got nasty swell, expect to crash & burn a few times.

And you can get at least a season of hard racing out of most spins, unless someone goes through it (or it snags on something) in a wreck. I've got one that's been through about 2 racing seasons (distance & bouy)and the 2001 (?) Worrell, but it is a bit on the baggy side....

I find that when I'm starting to get concerned about putting up the chute but I do, the boat smooths out and becomes better behaved with the spinnaker flying. Granted, this is up to a certain point where you still have some ability to head up and down in the waves.

You'll also find that with the additional speed, gusts (especially if sailing on a lake in heavy air) approach you more gently while you have the spinnaker up and give you some additional time to react. If you are moving slower, the gusts hit you faster giving you less time to respond. That being said, when you do finally crash and burn with the spinnaker up making 20 knots, it's a bit more exciting than without it doing 16.

I expect to get a year out of a spinnaker before it is semi-retired to backup status...I do about 18 regattas a year.