1) The default chest behavior makes inserters check each slot in a chest every time you add/remove items. Because of it large merged chests like this one or those in warehouse mods cause a huge performance (UPS) hit. In the forums there was a proposal to fix the UPS hit by internally treating the storage as one slot per item type, where the slot holds a single stack as big as the sum of the visually displayed would hold. Does your mod already do this to avoid the performance hit? Would yo implement it if you don't already do it?

2) Can blueprints be used normally when using your Modular chests?

1) I have not done anything to make my chests act any differently from regular chests. If there comes any official support for this, I am pretty sure it would just work. But I will not implement this myself, at least not in the near future, as I already have updates planned and started, but put on hold because of real life, which I would much rather prioritize if I get some time to work on the mod.

2) Blueprints only work when single chests (1x1 size) are copied to the blueprint, as you cannot obtain the items for longer chests through normal gameplay. I would like to fix this, but I cannot promise anything atm.

a) Could you make it so that it instead blueprints the merged chests as separate single chests? Understandable if not.
b) Could you make single chests placed by bots auto-merge? Currently they do not (assuming they were in a line and the correct length, etc).