I moved the other way, from an ST3 to a 999. On a long run you can move around on the 999 seat and have a different shape to sit on to relieve the numbness (happens after about 200 miles for me).
Although the ST seat is superbly comfortable you do eventually feel that it's too soft and not supportive enough - and when you try to move around there's no change to the shape, you just sink back into it and nothing has changed.
I'm wondering if more than one manufacturer is losing interest in the Sports Tourer market, e.g. Honda have again decided not to update/replace the VFR for next year despite the clamour from owners for something more competitive. I can't stand the thing myself, but it's not terribly long since it was a huge seller and Honda just seem to be letting it slide.