The best thing to use is a scrolling jig saw with a metal cutting blade. I enlarged the rear door opening in about 2 minutes per side to accomodate my Diamonds. Made a very clean cut...way better than a dremel or die grinder.
What a pain in the butt!! I spent about 3 hours and 3 broken dremel grinding bits working on just the passenger side rear. This worked our cleanly but it certainly wasn't an ideal cutting solution in terms of speed. I tried to use the cutting disks at first but I really couldn't find a good way to make a clean cut with those. I think I'm going to use the grinders and patience on the other side.
Does anyone have any thoughts regarding rust after the grinding is said and done?
Don't do it. I used a carbide tip grinding bit on mine and it took forever. Not only that, being the dope I am I decide I wouldn't need gloves for the job. Neways I ended up getting about 30 metal splinters in my hands. Not cool to have to dig each one back out with a needle. Plus after cutting one door my dremel is acting up now, even on slow speed it revs like its on high speed now.
Instead go with a tool you can get at radio shack called a nibbler. I found out about this great tool on the jdm sidemarker thread and realized it would work great to enlarge the rear speaker hole. Plus its less dangerous to use and probably will take you less time.
As for the exposed metal, I just took silver paint and painted where I cut.
The absolute best tool to cut the metal with is a SCROLLING JIGSAW w/metal cutting blade. You can pick one up for under $50 and still be useful in the future. It cut like cutting butter! Very fast and neat...