I found significant FC with the XW40 in my A-P 130 EDF (f/6). It was OK with my dob (10" f/6). But I wanted a widest-field that would work with both... so out the XW40 went (twice--I really wanted to like it!), replaced by a Pan 41 that renders outstanding views in both. Albeit at a cost in weight which is no issue in a dob, but annoying in the balanced refractor.
So no, I will not be buying either new XW--been there and done that.
I also had the opportunity to use that eyepiece in 80mm f/6 refractor, same result. Ten years later it still baffles me as to why telescopes with opposite Petzval surfaces showed the same result.
Supermonos have a reputation for field curvature. I owned all focal lengths from 5mm to 14mm using them heavily, and never noted it whether the scope was a 2800mm Newtonian or a 1000mm refractor.
Indeed, it is rare for me to see field curvature in any eyepiece. That is why I leaned toward the "defective sample" theory on the 40XW, as improbable as that is.
The 41 Panoptic is indeed a safe choice in that class. After a long series of eyepiece moves I eventually ended up with a 40 Paragon. Performance is quite good in my f/7 scopes, and seventeen ounces sealed the deal.
It would be nice to get a look at another 40XW, but I don't want to go through another return process. Probably the safest course would be to talk one of my observing buddies into buying one