Would a .40 cal pistol be sufficient to cleanly kill a deer with?
Kill a deer? Sure.
Good enough for hunting them? Nope.
My thoughts prezactly.
In the last 14yrs I've shot well over 100 deer and elk with a .40S&W from a Glock 22. They've all been easier, closer shots than you'd encounter while hunting because they were crippled by motor vehicles first. Long story short, the performance is not inspiring with anything other than head shots. Given half a chance I use a long gun instead.
In a former life as a state trooper I had very similar experiences. I was issued a G21 .45 and put down similar amounts of vehicle struck game. The .45 was instantly effective when head shots were used. When the critter was semi mobile and would drag itself away at a rate faster than I could keep up on foot, I would shoot for the shoulder and bust the front shoulder, putting them down so I get get close enough to put a mercy shot into the brain. At one point, one winter I was so frustrated with watching some coworkers shoot at animals, causing more suffering, I held an informal class with a couple on the anatomy of game animals. They were trying to do the right thing, they just did not know.
In regards to the .40 for deer. I know of a very small number of exceptional handgun hunters that I would be totally comfortable with them using a .40 to hunt deer with. These men are crack field shots and respect their game enough not to cause any undue suffering. For the vast majority of handgun hunters, I would say stick with a 44 mag or 45 Colt.