I have the 20mm and 15mm pairs. My 15mm pairs just arrived today. The 20mm's are excellent. I use them in my 10" Orion XTi with William Optics binoviewers. The field barely vignettes at all, sharpness is great right out to the edges, there is no ghosting on the moon or planets, and IMO they just rock !!! Eye relief is great, and they are "glasses friendly". Coatings are green and purple. Weight is a non-issue because they are so light! They are indeed Expanse clones @ 66 degrees.
I have yet to try out the 15mm pair in my binoviewers. So far, in just looking into the eyelenses, eye relief is good.
I have to think that you are using an OCA with your XT-10 if you are getting sharp to the edge fields. In my experience, at F/5, the 15mm and 20mm are Erfle like, at F/12 or so, they are quite well corrected. Your mention of 200x with the 20mm suggests you are working at F/16.
I purchased the 6mm, 9mm and 15mm Pro-Optic versions of these eyepieces when these first hit the market, that was in 2003. Since then, I have had the 20mm as well as several incarnations of the 6mm and the 9mm. Currently I have some version of the 6mm and 9mm.
The 6mm and the 9mm are quite well corrected in a fast scope as they're negative-positive eyepieces, in the barrel there's a Barlow-like Smyth lens. The 9mm has been my favorite but it does show significant EoFB. Still, it was a favorite eyepiece.
I believe the 15mm and the 20mm are some sort of Erfle derivative, they're just straight positive eyepieces and lack a negative section that provides the off-axis correction so they're quite good in slow scopes, not so good in fast scopes,
As an aside, Antares had their W-70 Series and there was quite a bit of speculation that the these were the Expanse optics in a different package. I had both the Pro-Optic 9mm and the 8.7mm W-70 and one Saturday, I got to fooling around with the W-70 and started to take it apart. What I discovered was that the 8.7mm W-70 was nothing more than the 9mm Expanse "clone" with an aluminum cover glued on.
It seems to me that the 6mm and 9mm Expanse type eyepieces were the first affordable negative-positive eyepieces. Soon enough, the TMB Planetary's and the Celestron X-Cel's came along. I think these represent good value and are decent eyepieces, particularly in a slow scope. One of these days, I hope to pickup the 20mm and the 15mm, in part to relive those days and in part because they are quite good in a scope like my Meade 310, an 80mm F/11.3 achromat with very nice Japanese optics.