I don't think that the pH of the sparge water will have any effect on efficiency, but a low pH can help prevent excess tannin extraction.
I increased by efficiency by 10% when I started doing a mash out. If you are not doing a mash out, and are fly sparging with 173F sparge water, then I doubt that the temperature of the grain bed is getting out of the 150's
If you are using a braid, or a manifold without cross bars, you are likely to get channeling with a fly sparge. This can result in the final runnings having a low gravity, but still leave a large amount of sugars trapped in the grain.