I had the same issue for awhile. Then it just happened to hit me. My ENO slap straps stretch... alot! Thus whenever I got into the hammock I had alot of headroom. Lately I have just been using some climbing rope and trucker knots. Problem has disappeared. Dont know if this pertains to you, but maybe it will help.
I tried rubber tensioners on the tarp ridge and found that it makes it very difficult to center the tarp over the hammock correctly because of the play in the bands. Of course, when I did this I was using the smaller stock tarp where it was much more critical. It may not be as critical with a MacCat.