Well, if it makes anyone feel any better, I've been using 5.2ph stabilizer for several months and have been above 67% efficiency, usually 70-73%. Just did a batch of stout last sunday, forgot the 5.2 stabilizer and my calculations are coming out that I only go 61% efficiency...
Maybe it's the grain I used, the oats, the toasted wheat....but only getting 61% efficiency re-enforces why I started using 5.2 stabilizer in the first place.
If you only got 3-4% extra efficiency, it's worth the price.
just my two cents.