I’ve never listened to the Kef,so I can’t comment on it. I own the P3 and have owned the 7. The p3 will work better in your size room. Here is a nice review on the P3. BTW, I have owned many LS3/5A type of speakers and the Harbeth is my favorite!
http://www.hifizine.com/2011/12/harbeth-p3esr/
http://www.hifizine.com/2011/12/harbeth-p3esr/