Don't just gut them. If you really want to try to fool people the right way to do it is replace them with just a pipe and weld the shell of the cat around the pipe that is there. Having the open space of the open cats gave me fits with my last mustang. The owner before me gutted them, it took me all of 2 weeks to pull them out of there.
i ran gutted cats just fine, if the cats dont work (mine had pieces broken off, rattling in the cats) just gut them. I actually had cops look under my car to make sure i had cats a couple times ($1500 fine). the only con is the exhaust will be louder (if you count that as a con) and you wont be able to pass smog.
I have thought about it a little and tried to find more info, but google is letting me down on this subject.
The main concern with gutting the cats is that it will create a large open chamber in the emptied out cat shell. This large chamber allows the exhaust gasses to slow down once it enters the chamber, and must be accelerated again when it exits the chamber. The theory is that it will be more efficient to have exhaust tubes of constant effective diameter. This is the theory I could find, not sure if I buy it or not though.
I would like to see some dyno results of with gutted cats; before and after of course.
Since your cats are obviously in good condition I would suggest not destroying them. If nothing else remove them and install an aftermarket off-road H-pipe. Then the next time you do have to pass emissions, you will have a good set of cats to throw on the car.
ebay should have some used offroad pipes fairly cheap.
Jason is right on. I wouldnt gut cats in the first place and esp not ones in good shape (I know you're north, but in King County we had to pass emissions many years ago).
I just picked up a Summit H-pipe for 100 bucks and it looks like a decent piece. Make sure to consider the metal and treatment of any pipe (including the one I mentioned) to deal with the wet weather up there.