Holy crap. I can just imagine the shitton spam of miners on every single tile. It would mine the total ore fields SO much faster. I mean, mini miners as 2x2 entities are already pretty OP compared to regular miners (pre-modules) since they put more ore on the belt (on average) and allow for more lanes of belt per field of ore.
I think the issues would be more about user interface than anything else. In vanilla, it'd be fine to have something like a Micro Miner, as there's only one tier; thus the crafting menu would only have [Burner Miner, Electric Miner, Mini Miner, Micro Miner] all in one line. But when you add in Bobs Mods, there's 4-5 new tiers of miners, and mini miners, and then micro miners on top of that... or replace the later tier mini miners with micro miners... but that would just be inconsistent... I dunno.
I definitely wouldn't do it for any other machine though. 2x2 entity is good for space conservation, if you only need to do small things; but for things like science packs and stuff that requires more than 3 ingredients, having more sides to insert and output items can be more beneficial than the amount of space saved.
