Although this may be possible, we always have to consider perception vs. actual diminishing returns. At first, AP or P-Slin blows your mind, and that initial difference stands out to you... of course you'll become accustomed to that after a while. I think that's mainly the case, at least I havn't seen a decline in my 1.5 years with AP and 4+ months with P-Slin.
It does take some trickery for some people to really reap the full benefits however.. just have patience with it and attempt dosing in different ways. Personally, which I'm about to do in any minute now, is dose my P-Slin 90 mins before my workout. T-70 I'll have my meal, consisting of 60-100g carbs. T-15, just before I get out of the car to enter the gym, I'll eat a banana/apple/orange. It may or may not be the absolute best idea for body composition, but it CONSIDERABLY raises my endurance output. IMO an underrated product, specifically because of the variance it has between individuals.
But I keep reading how people "don't respond." Last time I checked, there wasn't any samples of this stuff, so you gentlemen must be in possession of a whole box of these goods. Experiment with it, because it does work. (or just give that shat to me, area code 01602 oraahhhhhh lol)