All of the companies you've listed above try to make sure their parts fit with STOCK connections. As such, if made correctly, they should all bolt up just as well as the stock parts did. Realistically, most of the manufacturing is outsourced anyway and tends to be done in batches. Basically, the brand name doesn't matter much, just the quality of the part. Some brands have higher quality standards than others but mixing and matching brands should not be looked down on.
Also, a lot of the parts you're talking about don't even interact with each other. They connect to stock parts. The stock connections in between make discussing brand unity rather moot IMHO. Are cobb springs better for a car with a cobb intake rather than an SPT intake? God I hope not or else something is seriously wrong.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD