He's not saying a Tempaflow is not a "good" product. We have tried them, and I really like how they work. Hell, most people don't even keep track close enough to say if they get 15% better mileage. I do, and can tell you it did not gain 15% for me. It do what it is supposed to do though, it runs the same at all temps. Other things did gain about 50% improvement, on my current sled. I also agree it is not worth full price IMHO, so I've been looking for used ones, cheap.
I would also say, as said earlier, it will help mostly when riding in warmer temps., but only if your not one to re-adjust your low speed fuel settings. You can get the exact same results as without it, if you pay attention to tuning.
IMHO, Mileage is a result of many things being set right, any of it wrong and you won't find good results. Exhaust valve sticking or open too soon, is probably the biggest issue for many. TPS setting on many Polaris's can be a big factor. Low speed clutching is another big factor, great hole shot set-ups usually will kill MPG.
A few years back we decided it was worth the risk of burning one down to try and get better MPG, so we started leaning down and found much more too it than that, and every sled is usually different. The 700 ves are easiest IMHO, then the 800 ves. The 500 and 600 are a little trickier to get better MPG with.