As for measurments, there is not a single reason to belive it won't be in the bottom zone of ASR charts. That has been the case of most of the devices manufactured without expensive AP units.
I have a KGSSHV and can report that it is considerably better than any Stax amp I have listened to. The market for electrostatic amps is much smaller than for "regular" amps, so there aren't a lot of products out there. I think the basis for my love of my KGSSHV is it's additional headroom for driving electrostatic 'phones. My Stax SR-007 (original version) need a lot of voltage swing to work well, and if you are going to add the EQ needed to really improve them, you need LOTS of extra headroom over what a Stax amp can provide.
I suspect that the KGSSHV is also inherently more "transparent" than the Stax amps too, using better topologies to achieve lower distortion. Not in the same realm of the exceedingly low distortion and noise of things like the Topping amps, the Monoprice amp, etc etc, but still better than Stax in this regard. Dr. Gilmore has the instrumentation to measure THD at high voltages with a capacitive load, and he designed the amp to be low in distortion and noise. It is a "good" or possibly "very good" amp, but likely not in the realm of the stratospheric amps that users of dynamic and planar headphones can now buy.
Likely there is **NO** existing electrostatic amp that is as transparent as something like the better "regular" amps today ( like the ones with the THX modules in them ) but as far as electrostatic amps go, the KGSSHV is still head and shoulders above the stuff from Stax. The KGSSHV-Carbon and DIY-T2 are said to be a step up from the "plain" KGSSHV so maybe they have even greater transparency.
All this said, I like the sound from my SR-007's and KGSSHV better than any of my other headphone/amp combinations ( with appropriate EQ) - the next best are my Hifiman HE-6 4-screw 'phones driven balanced from a Monoprice Monolith THX amp, using Amirms's EQ. These are also quite good, but not as enjoyable as the Stax SR-007's. Not as transparent.
I almost sold my SR-007's before I bought the KGSSHV, they just did not sound very good on the Stax amps I used them with. After buying the KGSSHV I would sell all my other headphones before I would sell the SR-007's.
I am using a Schiit Modius balanced dac, FYI.