He never stated the Diamaris were bad. He was asking whether there were better options than the OEM tires.

What he indeed stated, was the fact that OEM tires are usually not as good as they could be. Which, in his own special case, could be very true.

We all know that the engineers go a great deal to test tires for these cars, but the choice is not only based on performance alone, but also on economics. Tire manufacturers work in conjunction with the car manufacturer to produce a tire for that specific vehicle. These tires, for the most part, are a great compromise, because they have to suit all types of buyers (in contrast to an owner who is looking for a tire that would suit his/her specific needs). The original tire manufacturers sell these tires to the car manufacturer for a hefty discount (pretty much at cost). Later, the tire manufacturer makes up its investment on people like you, who believe the OEM tire is the best way to go.

This fact, doesn't mean that other tire manufacturers cannot produce as good or better tire than the original one.

I have 3 M cars and none of them use the OEM tires. For my driving style and needs, the Michelin Pilot Sports 2 are a better tire than the ones that originally came with the cars. Which doesn't necessarily mean the "BMW engineers were wrong".