I have 5.7m Rib with the Mariner Opti 115.
With 4/5 adults, which is my usual loading, I get around 5200rpm and about 35-36kts.
However, with just 2 aboard, she will pull 5600+rpm and get 42kts. This is with the standard ali prop.
The biggest issue is the leading edge of the prop. If it has the slightest dink and therefore blunt edge, the rpm can drop on WOT. Take a bit of time to clean it up and a dab of paint and away she goes. I average about 1.3miles/ltr at around 30kts
Hope this helps