"Will say a 66-72" bow not reap any of sinews benefits?"
I used to dream of making a killer Osage ELB with sinew backing and having the world's fastest bow. Then someone explained to me by drawing a picture of a short bow pulled 27" and long bow polled the same length. (Or maybe they showed me the picture in The Traditional Bowyers Bible, I can't remember) The shorter bow has to move it's limbs into a much steeper bend to acheive the same draw length, consequently it "Loads" up more. On the longbow, it really isn't doing any work at all, it's just freeloading.
But if you put on a really really really thin layer as an insurance backing it wouldn't detract much from the performance while preventing splinters from lifting. But you could use antelope rawhide, which is tougher than steel and just a bit thicker than cigarette paper to do the same job. Ultimately, save your sinew for a shorty bow that will be so fast it will burn the feathers off your arrows!