Yeah, I thought about it some more and realized it worked better than I'd thought. It runs into issues with super-high Strength scores, but for PCs it's solid.
Regarding monster deadlifts, let's keep in mind that that's an Olympic weightlifter, whose training is focused solely on pumping the maximum possible amount of iron. PCs with 20 Strength are combination decathletes and martial arts masters; they can carry a lot, yes, but they also train to swim and climb and jump and fight and bust down doors.
I'd suggest having a feat or something to represent "can carry a lot," then calibrate the numbers so that when you put the feat together with Strength 20, you get Olympic weightlifter stats.