Give me a day or two to bring over my links from the old computer. I have done similar mods to motorcycle taillights, and had started on the WRX's lights before I sold it.
I have a ton of info on this mod.
Easy trick:
Use a hot glue gun to seal the surface of your board (on the LED side) so that you can paint it with the silver paint without worrying about shorting anything out. Also, you can just use white paint -almost as reflective and a lot neater.
EDIT:
Here's one example, I think his pics are gone, but decent tutorial:
http://home.comcast.net/~jbipes/ttr/ledtails.html
also, you can join nasioc (www.nasioc.com) -it's a Subaru group but there's some good info on "LED +conversion +taillight" in the search. I can't remember the guy's name, but he did an awesome writeup of a full LED conversion on his Impreza. It's archived and shouldn't be too tough to find.