I have been experimenting with disk images with mixed success. ProDos based system utilities disks work fine. No luck with Karateka though :(
I fixed a few bugs that were causing the program to not move to the correct track at times. Also, I found out that libmpsse stalling was my fault - I commented out some code to limit transfer sizes thinking it didn't look right. That was causing the UM232H to get too much data and stall out. Finally, I have been experimenting to see if I can reduce latency beween data transfers. Loading a large program is still about 10% slower than running the same disk on Virtual ][. Clearly there is some room for improvement here.
FTDI has Mac OS X drivers for the UM232H. I may test out their driver to see if there is any speed to be found there. Also FTDI recently released a simpler / cheaper version of the UM232H named the UM232H-B. $15 is a steal! You could make a Disk ][ emulator like mine for well under $20. Mine is still under $25.
Still awaiting resistors being shipped from China for my IIgs.