Yogiboy,
I think you would be fine with the Harbeth SuperHL5 Plus in your room. I am using the M40.2 in a room about the same size as yours with no issues. I have heard the SHL5 plus also in rooms similar to yours. They did not have to be too far out in the room either.
Have you ever heard the SuperHL5 plus before though? They have a very nice full range sound, but will have a much more extended top end than your P3s. Some folks do not mind that. Others have found it to be on the overly detailed side.
Might be worth a demo for you , if you have a place locally to listen.