So I finally figured out that the reason I wasn't able to exceed 19200 was because I am using a stock OS. I didn't realize that higher speeds are not possible without a patched OS. I do have one machine with the Warp+ 32-in-1 OS that achieves better speed.
Looking at Hias' notes, it seems like I should be able to run his patch at the beginning of a session and get better speeds on the stock OS. Is there a method for doing this? If so how? I have tried without success.
Also wondering if anyone with an EPROM or EEPROM burner would be interested in creating a couple of patched chips for me. Things like Ultimate 1MB are overkill for my use - I just want to get faster load times using RespeQt.