Audiophiles and wine-testing professionals can be quickly shamed with something very simple, called "blind testing" :). "butt dyno" was mentioned in meaning/context, as change to better or worse most probably cannot be felt and thus mostly product rating/evaluation, even if from own experience, plays little role.
And you are 100%, absolutely, swear on bible, sure that "quieter" & "run smoother" is not there because you expect it in subconsciousness and drive differently? For example, some exhaust bolt-on mods theoretically should add performance & let engine run more efficiently and should rise fuel economy .. but then again it's possible that simply driver's foot becomes "heavier", so to see that extra performance he paid for, and so to hear more audial pleasure he paid for, thus running more often at higher rpms or flooring more often, instead of granny eco-coasting and early downshifting, as result fuel economy drops, even though to driver it seems that he drove no different as before. Actual gain (or opposite) should be evaluated by leaving out personal/subjective bit and with trying to eliminate other variables. For example, that quietness/smoothness to be measured with actual sound level meter and something to measure actual vibrations of engine, with it operated in some mechanized/automated way for repeatability. Long term wear level - on larger number of engines (to reduce particular bad engine possibility) running with or without those additives, filled/operated/run in some specific testing rigs same way. One shouldn't undermine placebo feel/positive affirmation bias/purchase self-justification big role in personal/subjective tests. We are humans, it's our nature, it's known shortcoming of our psiche. We paid money for that, we expect it to enhance things, and we'd hate of opposite and feel remorse of finding out that money was spent for naught or if we need to admit own mistake. Unfortunately there are many less honest vendors/shops/institutions that are glad to exploit known shortcoming/trait to make money.