I also have a Kokatat short-sleeved spray top, but find that I just don't get a sufficient arm seal with it. When playing around in a hole the pressure will force water up the neoprene arm gasket. It appears I need a better seal at the arms than a non-latex gasket can provide.
As for useful window: I always wear my drytop except on the hottest of summer days, to keep the water out. I hate stopping to drain my kayak. Other folks have to get out of their boats every 15-30 minutes to drain, but with a good top I can keep going 2-3 hours straight. I've found that I like the comfort of the short-sleeved spraytop, though, so I'm looking for a combination of the two. Hence the possibility of a Gush Shorty.