Here's an article I wrote several years ago that principally applies to standard backstay system vs. 'headstay/headsail efficiency' ... but also (which I do) also applies to running backstays when youre looking to 'tweak the rig' and not to prevent/eliminate 'mast pumping', etc.
http://www.ftp.tognews.com/GoogleFiles/Matching%20Luff%20Hollow.pdf