I don't think that anyone said the Jade II amp was a direct physical copy of the SRM-001, just that the circuit was similar, in that both used an op amp input. And given the difference in form factor between the two, of course the Jade II amp can run a higher output current than the 001. The exact quote is, "the circuit is more like the SRM-001 blown up a bunch. So opamps input, solid state version of grounded grid output stage, resistor loaded, and capacitor coupled output." None of which implies a physical copy.
Now, assuming that the Jade II has a differential cascode output circuit (still no published circuit diagram so cannot confirm), this will in fact reduce the input capacitance of the cascode stack as HFM state, since the MOSFET is being driven via its source terminal rather than its gate, so the capacitance to drive the circuit is that of the bottom transistor. The input capacitance however is still the capacitance of the transistor, NOT the parallel capacitance of the two together, so the equation in post 8 above is still incorrect.
On the other hand, the output impedance of a cascode circuit is quite high, roughly the resistance of the drain resistor. This can limit the high frequency output of the amp circuit, as it is driving a capacitative load (the electrostatic headphone), whose impedance decreases with rising frequency. This problem can be decreased by some extent by negative feedback, so how much of an issue this is in reality remains to be seen.
Finally, 1200 mW = 1.2 watts into 10 kHz works out to 2.74mA RMS output, or 437 VRMS. I wonder if this is into an electrostatic headphone load, which is typically around 100 pf, as the Jade II circuit apparently uses resistor output loads with a 600V power supply, and the resistor loads generally limit the high frequency output capability of an amp circuit (for example, the Stax SRM-T1, which uses 66k plate resistors, is specified as 300VRMS output, and that at 1 kHz).