Plastic or aluminum prop

[5hpoutboard]

I have a 3.3 on my zodiac with a 6 inch plastic prop. It seems that the past 3/4 of throttle just makes it rev more but there is no change on the gps (8knots) it's like the prop is flexing and I just wondered if it would be faster with an aluminum prop of the same pitch. On the digital tach it's doing 5600 rpm