Tried it in my car - advanced timing by 8degrees with minimal knock.
I reckon for a BMW a mixture with Ethanol would be king. Since there is no ethanol in the recipe.
WRT to OB1 - also used it in my car with very similar results, OB1 does contain ethanol which is a plus - timing advance was lower than with PowaRon.
There are always horror stories about any product on the market, all octane boosters are sold with a use at own risk disclaimer.
I am a listed agent of PowaRon - full disclosure, and can tell you that I have had good experiences with both local octane boosters.