I never add phosphuric acid to my sparge water....but unless you're getting it way below 5.2, I don't think it's going to change your effeciency much. The number one factor to effeciency, barring inaccurate measurements, is always crush. Make sure your grain is crushed so that there's no husks that are intact.
On Tap: Barleywine, Coffee Oatmeal Imperial Stout (big big beer)
Fermenting: Double Simcoe IPA
On the Bench: Racer 5 IPA
"One of the first things early caveman did, when he crawled out of the mudd, was to make beer. And cavemen everywhere are still making beer...and drinking beer." - Jean Sheperd