I personally do not have a C extension, but it is possible to remove it mostly, I'd think. Since you must replace your current nut with a new one to put it on, you can remove the extension, and place on the old nut, or a new one. As for the screw holes, that's quite a bit tougher considering that there is no wood in that place anymore. I don't quite know how to approach that problem. I can think of several ways, but I do not work with wood myself. Maybe someone with some luthier experience would like to comment? Has this really been done before? I hear of them being put on all the time, but I've never heard of them being taken off.
As far as having a C extension goes. I'm in my senior year of highschool, and I never expected to see a part that needed it until I got into college or played in a community orchestra of some type, but we'll be playing with the choir and band for our christmas concert, and one of the pieces is Gustav Holst's Christmas Day. There is a section that goes to a low Eb and D. It might just be the arrangement that we have, but still.
I would still say that you do not need one quite yet. It was a rarity that I came across a part at the highschool level that has such notes at all. I'm just planning on octaving the notes, which you can do if you for some reason do find a piece. I do however hear that they're becomming more popular. My bass teacher has one, and uses it for a few pieces. So, really only go by need, and once you get it, don't take it off, you can still play the bass normally. Haha, after that long winded thing, I would say do not get one yet.