Boy oh boy!
I installed +4 Bosch in my 3.5 and it runs better than ever. I even have buddies with turbo charged cars and platinum is ALL they will use.
Some people have issues with plugs other than "originals", however I have not. Some older cars had resistor wires and non resistor plugs. Then one day the owner switches to resistor plugs and then notices a drop in top-end performance or poor idle.
Bimmers run lean and switching to a plug with a more intense spark can cause undesireable running as mentioned above. Why do people buy Platinum? Usually for performance right? That means that you are going to drive harder thus increasing combustion temperatures on a lean-burning, air-leak prone engine.
When going Platinum first make sure you have no air-leaks, install a better coil, and fer gosh sakes choose a plug that is one heat range cooler than stock.
On my 85 635 I got a bit more performance and improved my idle by making a simple adjustment to my ECU. In the ECU is a rotary switch that adjusts timing and fuel injector pulse width. I increased pulse width +2% and I have never been happier.
Anyone want my old Silbers? DieInterim