Having splitters able to stack does lower the barrier to entry to start using stacking - I could see people using them initially to consolidate the output of existing smelter blocks into fewer belts to squeeze more down their main bus footprint without a complete re-tooling of the entire production ...

TL;DR
Where a splitter has a filter applied, provide an option to tell the filtered side of the output to stack items up to the current level of belt stacking research.

What?
Add a "Stack Filtered Output" option to splitters that is linked to the Filter option. The "Stack Filtered Output ...