Alesser reason for the lower grade as well as the material costs (and you are quite right by the way, the lower the grade of Stainless the higher compliance to magnetic polar attraction) there is the cost and associated wear factor on the manufacturing tooling. Even if 304 were the same cost per tonne as 441 and vice versa, the car companies would opt for 441, as the bending and forming mandrels on a production nolume part, would wear out quickly agains 304.
Immagine you are Mr BMW, A) you like women with hairy airmpits, and B) you make 100,000 3 series exhausts per year, so tou tell the supplier that for your hard earned 10 million Euro. you expect 5 years tooling life. he designs the tool for 500,000 units, and then you say I want 304 instead of 441, he's gonna hit you for more tooling monez after 18 months.
Janspeed etc, make a far higher mark up, but onlz probably make 5,000 parts per year, so their tooling is far less sophisticated.
I tell you, the car trade is a huge con. It's like, If you say, you don't want, erm, aftermarket brakes or stereo or wipers, you'd prefer OE stuff, you aren't neccesarily getting the best Quality, you are just getting a more massed produced part.
I'll shut up now. Any one for beer