Quick update. On a cold start, the car idles at 1400rpm or so. When it does this, it also cycles the solenoid constantly. I'm not sure why it does this. Maybe it is to warm up the solenoid as well in cold weather.
With the way I have it mounted, even with the rubber grommet, the new solenoid is louder than the stock piece. I can't hear it with the stereo on or even when cruising above 40mph, but it did scare me for a minute when I first started up my car and I heard the clicking noise. For a minute I thought my engine developed a new tick.
I quickly found out it wasn't RPM depedent, though, and tracked it back down to the solenoid. Just something to keep in mind if you hear a new ticking noise from your car. All the more reason to mount it directly to a chassis bolt when first running it, you'll get used to how it sounds.
I'm glad people are finding this thread useful. It really does seem like a great, simple, effective DIY mod.