+1 I buy a lot of stuff from them. Mahle/Mann (OEM) filters are around 12 bucks each. Cheapest I can find (stealer wants $28 each for the same part)I use Pelican Parts. Their pricing is competitive in general they give you choices for each part.
My understanding is that the airflow restriction on any of the above is so low as to not make any difference. With dual air intakes on the M5, for a stock engine (ie, without headers etc.) there is more than enough air for peak performance....I'm wondering which is best - filter capability & air flow - Hengst / Mann / Mahle ...?