That's possible - but a bit bizarre (I'd be healthily skeptical about 'binning' even on large components like CPU/GPU cores or clock speeds - since the lower specs usually sell in greater numbers, so you'd need most of the chips to be failures? More like an agreed proportion of chips never get tested to see if they meet full spec). Apple already have to guess how many of each permutation of GPU cores & RAM size they need to make - if they made SD-size specific SoCs or mainboards they'd have to juggle 4-5 (SSD configs) x 2 (RAM configs) x 2 (GPU configs) x 2 (Max/Ultra) different SoCs/Mainboards through the whole manufacturing process. Much easier to fit the required SSD cards at assembly time. I suspect that's probably why we got removable SSD at all (also, if Apple were playing that game, the 512GB model wouldn't even have a socket for a second SSD).SSD upgrade restrictions on these might be a hardware limit. The controller may be specified for handling only certain amounts of flash or PCIe lanes at test time due to yield issues and the chips ”bucketed” and installed in matching capacity machines.
Much more sensible to have the controller configurable in firmware post-assembly - Apple can still block upgrades by not allowing the muggle version of System Configurator to change the setting.