Hi Brian,
I have used Zar Ultra on exterior wood but not on a S&G or strip. The ultra dries so fast you will have trouble keeping a wet edge. It will build fast with several coats and the color is rather dark compared to the marine varnishes that you mentioned. The Ultra will not self level as well as a slower drying varnish. If you wanted to use the ultra try inside the cockpit of the kayak or maybe the hull and then use the good stuff for the bright finish on the deck. That is my 2 cents.
Clayton Plunkett