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.
a: he has combat experience, used to be a kickboxer long ago. I remember reading that he quit because he didn't like getting hit in the head. It won't take long for him to get as good as Lesnar standing or on the ground, wrestling is another story. The strength advantage Mariusz would have over brock would be huge, that is if he continues to train like a powerlifter, I think the majority of people don't understand how strong powerlifters are, especially the abs, the lower back and the legs.
b: if he continues to take legal supplements that mimic hgh and anabolic steroids he will be able to compete probably until around 43 or 44, that's if he's really dedicated to the sport
c: whoever mentioned his gifts in grip strength is 100% correct, these guys do farmer carries with weights in excess of 300 pounds per hand. That kind of grip strength will make it impossible for a guy to get out of close range situations.
d: his chances of being competitive will come down to his ability to cut weight. I bet he goes down to around 290 and cuts 25 pounds to make 265.