What makes you think that factory parts are the best? GM is relentless in cutting costs to increase their margins since they have to compete with foreign and transplant auto manufacturers and carry their significant retiree pension and medical benefit costs which adds to the cost of their products.
My comments come from the perspective of a 21 year employee of a defunct automotive interior trim manufacturer that couldn't manage with what the Big Three allowed us to make. Every UAW contract came at our, the suppliers, expense.
Midas might actually use better quality parts than the big three. Their mufflers are better for sure.
The vendor with the lowest price gets the contract.
Wes