Great report guys.
It is a fantastic barlow.
IIRC the 27mm Panoptic vignettes in it somewhat (brightness drop-off approaching edge of field). This is probably specific to that Pano, because Dave 47tuc used his UO 2" barlow (same as the Orion Deluxe) with a 31mm Nagler no probs (see UO 2" barlow writeup under reviews). Maybe the TV Panoptic barlow interface is worth a try. Technically it should turn the barlow into a 2x Powermate - and they are not expensive either.
I've used the 17mm T4 + UO barlow combo myself and found it excellent also (and ridiculously heavy too). Lately I've been trying out the 20mm T5 in the same barlow and that's an absolute beauty too. Like you say, the 10mm XW has some competition now when more FOV is called for. But I think the effective FL of the barlowed 20mm is somewhere in the 9-9.4mm range, so the barlow works as a 2.1-2.2x with this EP. Ive measured the magnification more accurately with 13, 8 and 5mm Stratus EPs (used in 2" mode) and I got about 2.1x for those. If the barlow really worked as a 2x, your barlowed 17mm T4 should give exactly the same true FOV as the 10mm XW, but you will probably find it is a bit less.