DA,
Yes you have spotted the Canadian
I thought if I wrote CP-140 no one would know what I was talking about.
A mate and I were chatting about this question the other night and his thoughts were along the lines that the P-3/CP140 blade shape was designed to prevent critical airflow over the prop tips as he thought the props were turning faster due to the higher speed of the P-3/CP-140 ie 400+kts. Rather like a swept wing, but just at the tips.
It sounded good but I'm not quite convinced yet as I thought as you did about low level efficiency. I flew DH-6's and they had rounded tips too but I wasn't going 400+kts more like 150.