After having been hinted to subtle problems D2X-XL was having and causing during IPX (KALI) hosted multiplayer games with other Descent 2 versions, I finally managed to nail the bugs behind these down and remove them once and for all.
Basically, some game messages from D2X-XL users never were processed by non D2X-XL clients, making people think there was increased packet loss if D2X-XL clients were taking part in KALI hosted multiplayer games.
Actually this observation was pretty correct, it was just that the packet loss wasn't caused by erroneous networking code or bad connection quality, but by D2X-XL placing additional message types in the data packets leading to other Descent 2 version not being able to fully decode data D2X-XL clients sent them.
This is major milestone on the way to 100% stability and compatibility of D2X-XL. :)