(Geek squad here)
I'm thinking this would be a good experiment for the next meeting of the IPS -- do a higher-abv batch (say... 1.080) and pitch one with half a pack and the other with two. Take notes on
how long the lag phase is (higher with
lower innoculation count... could lead to infection), how vigorous/fast the fermentation is (higher with higher innoculation... supposedly
produces a clearer beer and blows off volatiles better), and ultimately how the final product tastes (lower pitching =
larger ester/phenol profile, more hydroxyl acid precursors which will produce VDKs such as diacetyl, and a greater oxygen requirement which will if left untreated lead to greater acetaldehyde production).
As for me; I have some recipes which seem to defy the logic. Black Like Your Heart got stellar reviews from people and at 1.070 is a rather "big" beer. I only ever pitched a single pack of yeast into that, and not rehydrated, which should mean it would be around a half a pack viable. By the numbers that's a serious under-pitch, but the beer has been great.
The scientist in me looks rather disdainfully at the idea that every strain of yeast would have the same "optimum" pitching rate; or every recipe of the same gravity for that matter. Personally; I feel that this is a recipe issue. Pitching rate is another parameters you have a handle on, or can tweak, in response to what you perceive to be a defect in a particular beer. If the beer tastes good - drink it - and keep pitching the same, unless you have a problem
hilighted above. The purists will no doubt rally against such anecdotally-motivated advice, but screw them - it's your beer.
My "hunch" is that S-05 has a lower pitching rate requirement than other yeasts.
fwiw, if anyone wants some seriously good reading on this, check out
Yeast: The Practical Guide to Beer Fermentation.