To answer the question posed on the thread title. In my opinion, theoretically yes, but I don't think Mariusz has the time needed to catch up skill & experience wise. I don't mean Mariusz can't catch up to Brock's "MMA" skills or experience, but his long established wrestling skills and competition experience. Theoretically, yes, realistically, Mariusz is definitely beast enough, but I don't see it happening.
Mariusz would definitely beat Lesnar. This guy is massively fast, and could literally be twice as strong as Lesnar. Maybe even more than twice as strong. Brock would have to bench 400+lbs, deadlift 500+lbs, etc to even be half as strong. Lesnar won't be able to push his weight down on him, he won't be able to hold his arm out (Mir II), he won't be able to ride him (Herring).
Lesnar couldn't keep Randy down, he won't be keeping this guy down. The core and leg strength of these strongmen is unshakeable. And this guy is known for his speed. A glass chin is all we could hope for, and Lesnar isn't exactly a good striker. I know I'm a Brock hater, but Mariusz is one the best athletes on this planet, and if he can cut to 265 he will be a force.
edit: Strongman DEFINITELY trains explosiveness and endurance. Maybe even more so than wrestling.
In interviews he seems like a nice respective but competitive guy, so MMA guys should really appreciate him instead of disliking him. He's been a world class athlete for years and has chosen our sport, it will be exciting to see this kind of guy transfer over.