New wheels are 17x7.5, OEM is 17x8.

Are there any downsides to installing hub centric spacers? Is there a risk of increased wear on wheel bearings? Amplified vibrations?

When bolted in place the wheel and spacer can be considered as a unit, so if the spacers are correcting an offset for different rims, there's no change in track and no problem. If they're being used to widen the track then the net effect is to move the rolling center of the assembly outboard, which puts off-center leverage on the bearing and can (not necessarily will) accelerate bearing wear or cause alignment/suspension issues.

On your 'side note' , Jesda I've learned BMW IS THE home of the finest sports sedan's......

The hub-centric rings definitely help when you can't find a small vibration. It's normally the culprit as long as the tire isn't flat-spotted and the wheel isn't tweaked.