First, there are numerous .223 silencers. Here are six models from AAC.
http://advanced-armament.com/default.aspx?pageId=23 Another six models from Gemtech
http://www.gem-tech.com/store/pc/5-56-MM-c9.htm Another one from AWC
http://www.awcsystech.com/products/suppressors/raider /
And the list goes on and on for silencers for .223.
Second, the 300 Whisper is not .223, it is .300.
And lastly you CAN suppress the SHOT of a supersonic round. You cannot suppress the sonic crack when the bullet breaks the sound barrier. You would be amazed at how much suppression you can get on supersonic rounds.
Hopefully you have gained a bit of knowledge on this subject.