I find that throwing the tiller extension over the transom and letting it trail behind during a gybe is best, don't have to worry about it then until you're starting to get going again.

I also find that adjustable tiller extensions are a pain in the butt, they always seem to slide at the wrong time, but maybe that's just me.

But agree with the rest, fast is best for gybing. Scary the first time but once you've done a good one, you'll never go back to slow gybes.