Interesting find. As you said the biggest gains are up to 1TB and after that it becomes moot.So I have been curious about this since trying to look into it more and I stumbled across this old Macrumors post.
https://forums.macrumors.com/threads/does-ssd-size-affect-speed-in-the-pro-max-mbps.2321863/
It appears that the SSD size does affect speed in the MBP and from what I can gather this is because of the bus that are on the various size SSD, but at around 4TB it is pretty much capped with the biggest differences starting with the 1TB and as you go higher the differences don't seem that much greater. Going off the chart that is included in the post, it seems the 1TB has a decently greater write speed, but it would be up to you to determine if that much more money is worth it just for a little faster writing which technically slows down a little more the more you write to the SSD.
Edit: The charts were also with the M1 Pro and M1 Max MBPs too which could also slightly make a difference.
Very interesting results - thank you for sharing.I actually purchased both a 512 GB MacBook Air and a 1 TB MacBook Air this weekend. (I returned the 512 GB model.)
Blackmagic results:
512 GB
2480.3 MB/s WRITE
2930.2 MB/s READ
1 TB
3039.6 MB/s WRITE
3012.3 MB/s READ
I ran it a few times, and the numbers were a bit all over. The WRITE speed for the 512 GB laptop always seemed to be the lowest number.