I have not noticed any differences in perspiration between down and synthetic; and I've got a bunch of time with both and I too am a sweater. Weight and bulk are really the only factors in my mind. I don't worry about the drying thing because I work very hard to keep my quilts dry, regardless of if they are down or syn.
While I've not used my Shenandoah as an underquilt, I don't think I'd want to try 30F with it as a UQ. Somebody has probably done it on accident, but I don't want to try it on purpose.