The .243 is capable of amazing accuracy, which is obvious from the target pics posted.
In my (sometimes) humble opinion, it is an excellent whitetail cartridge, in the hands of an experienced hunter. By that, I mean a hunter that is disciplined enough to only take a good shot.
A 100gr bullet is pretty light, and will probably not be as effective on a marginal shot as a larger caliber would be. Of course, this depends on what bullet you are using... a premium Nosler or Barnes will even the odds somewhat, but the regular Wally world ammo would be a little overmatched on the marginal shots.
That being said, I know that many hunters kill deer every season with the .223, and it's not nearly as good a deer cartridge as the .243.
I wouldn't feel undergunned with one, at all.