I have no experience with the original UHU design, only with the HP-UHU. I've found that ~130kHz is about max what my setup can handle. The docs on the UHU chip is a little vague, in one place it says 100-150kz and in other places it syas it possible to reach 300kHz "depending on settings". I've found the first statement, 100-150kHz to be right on the money - with the Multiplier setting set to 0.
With that being said I had major problems at the get go with the differential interface on the HP-UHU, does the original design have that as well - ie the SN75115 integrated circuit for the differential encoder interface? If it has, try removing the two caps connected to pins 4 and 12 of the SN75115 and see what happens.