Berny,

I went this route on my Inter 17 and it did not work. Having the kite right is so much more important. I just set the mainsail to roughly where it should be (always erring on the side of less tension than I think I need) and I will then lay the mainsheet on my lap/leg once out on the wire in case it needs a quick extra little tug one out and really heated up.

If the rudders are cavitating, the problem may well be there. Do they cavitate when you are trying to bear off, do you actaully need bigger or deeper rudders. Might you also consider the pole as well. Are you on Class Max pole length. I find that my Stealth (max class pole length) almost bears off automatically when a gust arrives.

I now (on the F16) cleat the mainsail and constantly play the kite. It never really stops - hard work, but fast. I also have a flat kite and this may also be an issue. Are you over tightinging the Spi halyard, this WILL pull any shape forward and make the kite collapse very quickly as there is a big change in camber up front.