I've not seen anything that makes me believe (for a moment) that Kas'im has anything close to the command and mastery of the Force (both sides) that Count Dooku possesses.
When has Dooku ever shown the ability to protect himself from attacks that destroyed a massive Temple that is 20 meters tall and almost a hundred meters wide? PoD' clearly stated that Kas'im was able to protect himself from Bane's Force attack. We also know from PoD that Kas'im very strongly values Force Shields and his has shown itself to be strong enough to stop anything Dooku could through at him.
Dooku is Yoda's greatest protege, and one of the very, very, very best in the Order's history. If I were to speculate, I'd say he'd be in the top ten of its greatest students.
Top ten Jedi ever? Perhaps. If you include the NJO Jedi it's less likely that he's in the top ten. Regardless, it doesn't make him stronger than Kas'im.
He served under Yoda and Sidious, who mastered the Force in a far more powerful way than anyone in Kas'im's time did.
Is that so? Sidious mastered the Force to a greater extent than anyone in Kas'im's time by DE, but not as of when Dooku was serving him. Same with Yoda. The difference isn't great and certainly doesn't make Dooku better than Kas'im.