From the game Mahomes returned vs Ten, the Chiefs have averaged 31.7 points and 376.4 yards a game. Against the Patriots they scored 23 points and gained 346 yards.
And they won with some significant help from some godawful officiating that cost the Patriots a touchdown.
The point is: a quality defense has a chance to slow them down.
I guess, but the 49ers play defense a lot differently from the way that the Patriots play defense. Only five teams blitzed more often than the Pats defense in 2019, whereas only three teams blitzed less often than the 49ers. Pats mostly played man, while the Niners play mostly zone. Also, the Chiefs offense really hadn't been clicking on all cylinders going into that Pats game. Past performance is no guarantee of future returns but, uh, that's definitely not true heading into this game.
Niners are that rare team who can get consistent pressure without blitzing, because their D-line is just so good. Only Pittsburgh had a higher QB pressure rate than the Niners, but Pittsburgh blitzed 37% of the time, versus San Fran blitzing only 21% of the time. Their D-line is the engine that fuels their defense.
On the flip side, Mahomes is excellent at dealing with a pass rush and is one of the hardest QBs in the league to sack; only Drew Brees had a lower sack rate. The Chiefs have an O-line that, for all of their struggles in the run game, have mostly been solid in pass pro. So, it's strength vs. strength, really.
I expect the Chiefs to throw the ball a ton and go up tempo, to try and wear down the 49ers pass rushers. Niners will want to rotate those guys in and out, so I wouldn't be surprised to see KC go no huddle at times.
All of that said, the Chiefs could handle the Niners pass rush, not turn the ball over, be able to consistently move the ball and score points, and still end up losing, because the other side of the ball creates some problems for them. I honestly wouldn't be surprised if we get a bit of a script flip, with the 49ers making some explosive plays and having some quick strike scoring drives, while Mahomes takes what the Niners defense gives him and leads some long, clock soaking drives. I think the Chiefs will be able to move the ball but Mahomes can't force the issue downfield, as I don't think anybody was better at limiting big plays than the 49ers.
I don't expect Mahomes to force the issue downfield, because what separates Mahomes (and all of the best QBs, really) isn't just his ridiculous ability to make plays, but also his ability to avoid bad plays. He rarely takes sacks and rarely turns the ball over. Jimmy G, on the other hand, has had some issues there. Whereas Mahomes was 2nd in the NFL in both sack rate and INT%, Jimmy G was towards the bottom of the league in both, finishing tied for 19th in sack rate and tied for 25th in INT%. And I still keep coming back to that as being the difference in this game. I think it's much more likely that Jimmy G makes one bad mistake that turns the game, whereas Mahomes almost never makes mistakes. In a game that I expect to be close, the team that makes the fewest mistakes and wins the turnover battle usually ends up winning.
I will say that if Jimmy G plays a clean game, the 49ers will be very tough to beat. I just don't expect that to happen.