I don't know what speed you'll start to get flutter. I try to keep my UCD around 1/4 throttle unless it's pointed up or for short bursts while doing aerobatic maneuvers, in fact I don't think I can remember that last time the plane flew flat and level for more than a couple of seconds and I always cut the throttle to idle when ever the nose is pointing down.

I did see a one of our club members flying his UCD with a 46AX and he started to get flutter during flat and level flight at 3/4 throttle. I'm sure your 55 makes more power that the 46. If your using a 12x4, I think a 13x4 or 14x4 is worth trying. I do think it's a very good idea to seal the hinge lines but I don't think it'll to any good without HUGE amounts of throttle control.