Doing the math, you're probably going from 5 to 8-9 platters, so 2.2-2.5 areal density, and a naive square root suggests it should be about 50% faster. So maybe track width is taking a fair share after all?
Though when I look at more sizes, it seems like 12TB drives were also hitting 260MB/s and speed has almost entirely stalled since, with 24TB being only a hair faster.