I don't believe speed is always the answer. You have to weigh all the factors. That is why there are so many different centerfire chamberings. The 220 Swift - probably the fastest cartridge ever developed is absoulutely no good for dangerous game up close. And hence, a 458 Win Mag is not the best choice for pronghorn antelope at 400 yds.
I feel its the same with MLs. I would not hesitate to use a 180gr .40 cal saboted bullet out of an accurate 45 cal ML for deer out to 200 - 250 yds. But if I were face to face with a big bear at close quarters, I'd want a min 50 cal with a big hunk of lead coming out of the muzzle.