I drove the car for about 30 miles after I installed those plugs today. I'm aware of a certain placebo effect when judging and comparing new and previous performance, but that's where Ms. moribundman comes into play, and without knowing what I've done to the car, she will tell me what sounds or feels differently. That way I can double-check what I perceive.

To make it short: Subjectively, the motor idles smoother and more quietly with the Beru Ultra-X spark plugs than with the NGK plugs. I can't hear the engine at all with the windows closed at idle, and I can barely feel it running. Objectively, the (warm) idle speed is now between 640 and 680 RPM, while it used to be between 640 and 720. I don't notice a difference when driving the car.

Now I'll have to see if there is any noticeable effect on fuel consumption.

That would be just the excuse I'd need. However, given my financial situation, I hope I won't be losing any parts. I'll soon need a new timing belt (etc), brakes all the way around, new O2 sensors, and I'd like to fix my A/C. I hope Ramen noodles taste better these days! ;-)