No it not new just not alot said about it.
I had to call Sierra to get info on it because it was listed in their catalog but, not on their website. You are right about the BC on it. Dave King has had quite abit of experience using it.
Most don't use it because it has a lower BC than the 300gr. Nothing wrong with it though.
The techs rave about the performance of the 250 Gameking. Mike even told me it has been used sucessfully on Cape Buffalo. It shoots tiny groups in my rifle now, so I'm reluctant to change. The 300's are just too slow in my stock .338 WinMag. At least I think they are.
Sheldon if you check the search section on SMK's you'll find that the performance is outstanding using these. Most non believers say that you can't use a match bullet for hunting but, sometimes just the opposite is true.
You say that the 300's are too slow. Have you tried other powders,brass or primers? All these things play an important role in getting the results we want.