The text of your post asks about the 338 whisper, but in the body of the post, you say the 300, which one are you asking about?
Since you mention the AR-10, I imagine you are talking about the 338, which is based on the Rem br case, which is a 1.5" .308.
My buddy Dennis just had a 338 Whisper for his contender, but only put a few rounds through it, not sure about the balistics.
I think the primary reason it isn't popular for the contender is there aren't suitable hunting bullets for it. With the 300 whisper, folks are using 30 caliber varmint bullets, that perform well at reduced pistol speeds.
In the 338 whisper, most all bullets are designed for the 338 win mag, and simply won't reliably open up at pistol speeds.