I disagree. If it only took one pellet in the head/neck to kill the turkey then your statement is more accurate. However, I would point out that lethality is associated both with the kinetic energy and penetrating ability of each pellet, and the total number of such pellets striking the turkeys head and neck.

In your example, the pattern of the 1 oz load will start to loose density earlier than the pattern of the 1.5 oz load due simply to fewer pellets in the pattern.

Hypothetically, wouldn't you agree that 7 pellets hitting the head/neck of a turkey at 45 yards has a better chance of cleanly killing that bird compared, say, to two?

This is the reason heavishot #7 is gaining popularity. It will penetrate 1/4 inch plywood at forty yards and there is a ton of it in a 3.5 ", 2.25oz load. I've cleanly killed turkeys with it at ranges most people wouldn't believe.

So, respectively, I strongly believe that more shot is more leathal, all other variables being equal of course.