Apple must have been targeting Thunderbolt 5 specs when designing the M4 generation of chips, in which TB5 is “only” still on PCIe gen 4 for data. Not saying Apple couldn’t have used gen 5 for the internal storage, but it’d complicate the design so they probably will leave that work for M5 or even M6 down the road. Especially if and when a new Mac Pro happens, there are PCIe card slots to care about as well.
On the Mac Studio storage cards, the biggest sequential speed increase has always come from maximizing the number of NAND chips on the two daughter card boards, and the upper limit has always been 8. With the M3 Ultra having a 16TB config, many speculated may be the limit had been raised to 16 NANDs, but turns out it is still 8, Apple just starts using 2TB modules instead. So the end result is increase of capacity but no increase in seq. speed.
AFAIK, 2TB, 4TB, 8TB, 16TB configs all use 8 NANDs. In other words once you reach 2TB you are already at the higher end of possible seq. speed in the context of storage channels, the extra speed you can get from real world usage of the 4/8/16 comes from leg room in each NAND cell.
