This. And if somebody doesn't like to search around: the cargo space calculation is the harmless part. The real power is when you look at hull reinforcement packages. And consider that the module splitting would allow you to apply full engineering twice in one slot now. So you can stack double the resists, even before considering that smaller MRPs have a much better value per slot size and mass than the big ones. (I mean, it's already in the OP. So you are aware of the issue. You do know why this can't ever happen. )
I also at some time wish i could split slots, as it would give me some freedom in setups which we currently don't have. But when considering what one could do with such a system change and how quickly doing things the new way would be seen as essential, there's nothing to really be gained. It would be just yet another level of power creep.
There's a couple of quite easy solutions to the imbalance of the smaller modules compared to the larger ones (and not just combat ones, stacking vast numbers of class 1 limpet controllers would be stronger than single larger ones).
Firstly, they could actually fix the completely borked scaling on many modules. When you are seeing linear performance increases on exponential size increases, there will always be issues. If a 3A limpet controller was actually about 4x as good as a class 1A controller and a class 4D HRP was close to 2x as protective as a class 3D HRP, then there would be no outright performance gain from splitting slots. Even if they don't get equal scaling to their size, they just need to scale as strongly as the split slot capacity would be in order to keep it balanced.
Secondly, they could only include slot splitters for the very large modules. Many module types max out at class 5, so why not only allow splitters for class 6 and above? These splitters would then break down these massive module slots into a number of class 4s and 5s. This could mean that a class 6 splitter would create a class 5 and a class 4 slot, a class 7 splitter would be two class 5s and two class 4s, while the mighty class 8 would yield four class 5s and four class 4s. This would prevent the stacking of tiny modules while also giving a bit more flexibility for the larger slots.