So in terms of max sustained output, the KG amps don't have much on the Stax amps, but what about max transient outputs? Can upgrading caps alone help in this regard?
Are you referring to high frequency transients or are you referring to higher voltage on a transient basis. In terms of the former, the ability to deliver an undistorted high frequency transient at a high voltage is dependent on an adequate slew rate. That said, most studies of the slew rate of musical signals suggest that the ability to deliver an undistorted sine wave at 6 kHz is sufficient to reproduce almost any musical signal, although the late Peter Baxandall reported that he found one musical excerpt that required enough slew rate to reproduce a full power 15 kHz sine wave.
Since electrostatic headphones resemble capacitors, this means that amps with a higher current output stage have the potential to have a higher slew rate, because the equation describing current required into a capacitor load is:
current (mA) = 2000*pi*voltage*frequency*capacitance (in farads).
So, for example, into a typical headphone resembling a 100 pf capacitor, 400VRMS at 20 kHz requires a max current of about 10 mA RMS, or about 14 mA peak. A T1S with constant current load mod can provide about 400VRMS but the output stage only runs about 9 mA, and some of that is needed to drive the feedback resistors, so only about 5.8 mA of the total output stage current is available to drive the headphones, which means it will go into hard slew limiting driving the headphones around 8 kHz, and it won't sound too good doing it as amps never sound their best when driven to their limits.
The Stax amps run between 9 and 14 mA/channel quiescent current output stages. The KGSS ran around 16 mA,/channel, the DIY T2 around 28 mA, the BHSE around 36 mA and the Carbon up to 40 mA/channel, so the KG amps definitely have higher slew rate. However, note that in most cases this potential is not needed because 1) most people don't listen at these high levels, and 2) most music has less slew rate requirements. In other words, most people will not approach the slew rate limitations of even the Stax amps.
The other option that you may be talking about is having a higher voltage limit for transients. A prime example of this is the original NAD3020 which was rated at 20 WPC continuous but had a "soft" power supply which allowed it to reproduce transients at a higher power, somewhere around 40 watts IIRC. However, this is because the power supply had a higher quiescent voltage which dropped under load to a lower voltage, and this occurred because the NAD was a class A/B amplifier, meaning it drew more current from its PS under continuous conditions (i.e. putting out a high power sine wave) than under music conditions.
However, all the Stax amps and the KG amps are class A, which means that they draw the same amount of power from the PS whether they are reproducing music or not. Moreover the KG amps also have regulated power supplies, so the voltage is further stabilized. So adding extra capacitance won't really change the transient power of either the Stax or KG amps. All of these amps are differential circuits, so current draw is pretty constant anyway (if the amplification devices were perfectly matched there should be NO current variation at all up to clipping).
and when he/she achieved this, he/she will know and understand from paper to personal experiences as of why (many things happens, technological and sound performances...etc..)
