I've never owned or ridden an Ultra that has been lowered with a lowering kit. Take a tape measure to the lowest point under the frame and measure to the ground; subtract out what the lowering kit reduction rate is and what have you got? Not much left. Factor in the loss at the floorboards and, at least for me, that's not the route I would take.
I did install the Profile Low Rear Shocks and original version of the Reduced Reach seat. The new-for-2011 Reduced Reach will place you even lower than the one I have, and then you have the Super Reduced Reach, which I think might be 2 1/2" lower than the OEM seat.
I could also install the Profile Low Front, but not sure if I would like the stiffer front-end and reduced frame height. Got to watch pulling in/out of driveways and those shopping center speed bumps, especially if you are two-up.
As far as 'is it okay', sure, it's your bike. There are a lot of lowering kits out there, so manufacturers must sell a lot of them. It's just a matter if you'll be happy with the compromises of the ride and lowered frame height; you could always take the kit back off.