James if I remember right, the 6.8 SPC has a velocity of about 2600 FPS. As Frizz said, the the 7MM-08 will out preform the 6.8 hands down!
As for the .243/6MM fading out, I seriously doubt that! It is a great starter gun, and doubles very well as a varminter. Whitetail deer are not all that hard of a kill, so the .243/6MM's with good 100gr. bullets is a very good minimum caliber for deer.