I have many customers come in to the shop with metal D loops on their bows. I really don't like these things. The two main ones I have seen weigh in at 37grains and 80grains.
I was working on a customers bow yesterday that had a whisker biscuit on it and he mentioned the fletching was tearing up his rest. I looked at it and said it's not the fletching, it's the metal D loop coming all the way in and hitting it. The rest was all dinged up and so was the D loop. This was on a 7" brace height bow!
So I talked him into changing it to a standard Loop and before we started I walked to the Chronograph. We gained 10fps by changing to a rope D loop. This bow is only 60#. I wonder if it would have been more or less on a 70# bow. I wish I would have Chrono'd the bow with the 80gr D loop on it. If 37gr got me 10fps, 80 could have been 25fps or more.
My advice DO NOT USE METAL D LOOPS. They cost you speed and can damage your bow or at least your arrow rest. I took some digital pics and hope to get them posted soon.