I have both cards at home now. I have just finished 3 days of testing.
If you were running the Jcat Femto USB card without a quality external PSU, switching to JCAT USB XE will be a true game changer.
Much better resolution, more transparency, deeper soundstage, much more palpability and more vivid colors. Jcat Femto USB card sounds almost gray in comparison.
It seems to me that the voltage regulation on the Jcat Femto USB card is the Achilles heel of this card. Throw in a quality ultra low noise PSU and it really picks up the pace. I have tried (discontinued) Jcat / Bacoon 02 battery PSU, Teddy Pardo ULN PSU and the excellent Jcat Optimo Nano inline voltege regulator. All of them sound a bit different - with battery PSU and Teddy Pardo giving more mellow but less finely resolved details and Jcat Optimo Nano offering more transparency - but they all move the Jcat Femto USB card to a completely different league.
That said, even with the excellent Jcat Optimo Nano - which is LT3045 based inline voltege regulator - Jcat Femto USB card is not as good as JCAT USB XE powered straight from the MoBo. And when you throw in Jcat Optimo Nano on JCAT USB XE, it sounds even better, although the change is not as pronounced, so you get only maybe 5-10% extra (which just shows, that local voltage regulation on the card itself is much more important than the voltage regulation before the card).
If I was to give a score to all of those card combinations (just to give you an idea of the magnitude of differencies), it would look like this:
MotherBoard USB port - 1
Jcat Femto USB (no PSU) - 2
Jcat Femto USB (powered by Jcat battery / Teddy Pardo PSU) - 6
Jcat Femto USB (powered by Jcat Optimo Nano inline voltage regulator) - 7
Jcat XE USB (no PSU) - 9
Jcat XE USB (powered by Jcat Optimo Nano inline voltage regulator) - 10
All tests made on Intel DN2800MT MoBo (CAPS V2) powered with linear UpTone Audio JS-2 PSU, running as Roon end point on Roon ROCK software (with Roon core on a different computer).