Dont get me wrong, i agree with your assertion that shot placement is a prime factor, but if it was the only factor we would all be hunting with FMJ's.
When it comes to quick kills, bullet performance is a factor. Period.
I hunt a lot in temps below 0F, and I do see less blood in the cold. As Lance pointed out, it's unlikely the bullet is effected by the cold - not only is the bullet heated by friction in its path down the bore, but copper and lead do not get brittle like some other metals do in the cold (look up DBTT - ductile to brittle temperature transition - this is well understood in materials engineering)
Hunting in -20C is not uncommon for me. Coldest kill I have is a wolf right around -40C.
I think there might be a physiological effect where wounds have a tendency to seal up faster and leak less in the cold. Maybe there is more tension on the skin to make hair bushier and thicker? Maybe blood freezes quickly as it exits? Maybe blood is a little more viscous due to cold temps? I don't know...
Yes, is unfortunate that some blame their failures on bullet performance without evidence, but I don't see that happening in this thread.
The issue of clogged hollow points causing bergers to fail to expand is well known. I always check tips. One member here has a lot of experience with Berger's and he just blows across the tips. If they dont sound right, he marks them for practice.
Haven't tried that method myself, but might give it a shot going forward. Seems easy enough, especially dealing with pocket lint or dropped ammo in the field.
Just my thoughts - not trying to stir the pot, sorry for the ramble
Just my thoughts,