Based on Football Outsiders' DVOA ratings (explained here), San Diego ranks 16th in defense until it hits the 20-yard line, and then it becomes the league's worst D in the red zone. The Chargers are only slightly better in the conventional measure of red zone efficiency, giving up touchdowns on 60 percent of red zone drives.
Kansas City's offense works the opposite way: The Chiefs rank a dismal 30th in DVOA on the first 80 yards of the field, but are second in DVOA inside the red zone. When it comes to turning drives into touchdowns, they are just middle of the pack at 50 percent. However, that conventional measure doesn't account for the fact the Chiefs haven't turned the ball over inside the red zone and have yet to run a play inside the 10-yard line that lost yardage. When they don't get the ball in the end zone, the Chiefs at least set things up for a high-percentage field goal try.
No matter which measure of red zone success you use, the Chiefs' offense is much better inside the 20 than it is overall, and the Chargers' defense is much worse inside the 20 than it is overall. That would seem to suggest a big advantage for Kansas City inside the red zone.
But what if there is no such thing as a team that is better or worse in the red zone?