As always the biggest problem is that the Bane saga did a poor job in some areas of giving real context for the overall level of the dueling and Force skills of almost everyone in the era. Considering the description of Kas'im's fights I'd say he had great prowess in saber combat, but I don't think it rises to the level of what say Maul does in Shadow Hunter against the training droids and such or the Jedi he faces, but it's all murky.
You have the premier Sith duelist in an era in which they are almost constantly at war with the Jedi, but you also have a period in which it is stated more than once that the dark side has been weakened by excess, in fighting, and failing to cull the lesser of their ranks. I'd argue from the descriptions that he could have killed anyone short of Windu, Sidious, Maul, etc, but we really only have a good showing against a younger, still developing Bane.
As to Force abilities, he was able to shield himself from one of Bane's blast, but I don't recall anything indicating that he was superbly adept with this use of the Force. I wouldn't rate him very highly except perhaps in being able to defend against direct attacks, and for how long is sketchy.
I do hope that there is an actual film Trilogy of the era and the Sith War so that there's a better measuring stick for the future.
The only thing I'll add is that the description of him as possibly the best duelist ever in a period after Tulak Hord had been in existence does give some credence to his capabilities. That said, there are only bare descriptions of Hord's feats as well and allusion to what he was capable of being beyond the scope of pretty much all those up through the Revan era, but no concrete showings. There's too much murkiness around all of it.
I think anyone trying to out and out dismiss Kas'im is being disingenuous, but anyone saying that he's at 'X' level without question is hard pressed to back it up as well.