I have tried various cap values as well, and today I made a comparison of both cap values and changes to the expo converter.
The expo converter core is dependent on a reference current. Right now that is set up using a 1.5MOhm resistor, but changing this does not change the output curve, it only changes WHAT linear CV gives what output.
I tried swapping for a 1MOhm resistor, and using the base octave trimmer I could still get the necessary output range. Here are the currents through a single I_abc resistor:
Using 1.5MOhm resistor, the output current is:
Full trim and 5V CV range: 3.2nA to 1.377mA
Center trimmed: 165nA to 168uA
Using 1MOhm resistor, the output current is:
Full trim and 5V CV range: 4.9nA to 1.379mA
Center trimmed: 248nA to 238uA
The center trimmed range is well within the total range independent of resistor, with room to spare for additional octaves.
The Juno 106 uses 240pF caps, other Roland synths use 330pF, and we use 270pF. Here are the responses to a center trimmed CV using a 1.5MOhm reference resistor:
0V = 7.7Hz
5V = 6.4kHz
10V = 42kHz
0V = 7Hz
5V = 5.7kHz
10V = 39kHz
0V = 6.1kHz
5V = 4.7kHz
10V = 34kHz
Note that at 10V all of the outputs are at their max (I_abc is 1.38mA) so tracking is off. But all filters reach well above 20kHz. If one aims for a 40k cutoff 240pF and 270pF seem equally suited.