Are you running a 5GHz-only SSID, or are you limiting the client to 5GHz via the .ini file (disableband=2.4G)? We've found that disabling the 2.4G band on the client has helped roaming also, presumably because the client isn't wasting time scanning the 2.4 channels.
Contrary to most, we've also found that the clients behave better with roaming aggessiveness set to Medium (-70dBm). High (-60) had the client in a nearly constant state of scanning and unless the nurse is literally running with the cart, medium seems to keep up fine.
This all being said, the thin clients we have (3030's) still aren't the most accomplished roamers and lag can be an issue for Citrix. I'm going to do some testing with 802.11r enabled to see if it helps, although I'm concerned about negatively impacting older clients that use the same SSID.