Just to close the loop on my original post, I ended up going with the Brooks Brothers BrooksStorm coat in Navy. I got it when they had their 40% outwear sale before Christmas.
They only had my size in extra long, but it worked out pretty well as I have long legs. As a result, the coat sits just slightly above my knee, which is what I was looking for (i.e. something longer than the car coat I have now, but not a full-length trench coat). The arms were a tad long, so I took it to the tailor for shortening. He thought they were probably close enough, but I got him to shorten them a little bit anyway; it would have just annoyed me every time I wore the coat. I ordered my standard size 42, and there's definately enough room for a suit under it.
The Loro Piana fabric is very nice to the touch; I'll be interested to see how it stands up to wind (and water).
At full price, I don't think I would have gone with this coat, but at 40% off, it seems like a decent option. In fact, I think that's my take on everything at Brooks Brothers. If you can get it on discount, their stuff can be pretty good. However, I don't know if I would ever buy anything there again at regular price, especially knowing how frequent their sales seem to be.