Alex, i've used your medium radius sill brush for a while and love it. The curve of the brush not only allows easier cleaning of the sill but it's also good on frames, cleaning multiple edges at the same time. Also the curve allows you to clean at different angles on the glass and still be effective.
Then I purchased your superlite medium dual trim and love that too but for different reasons. It's longer bristles and wider stock are quicker over the glass and good for getting into corners and dealing with those nasty spiders. Also good for getting under the fascias when doing gutter cleans.
So how about a combination of the two?
A radius sill brush about two inches wider (stock width) with longer dual trim outer bristles? And naturally as light as a feather!