Drop down to a 19 pitch highfive and I'll bet you'll see close to 40mph @ 4500 on the top end. High five is a thin blade 19pitch prop that gives great sternlift and is super smooth. The hubs kits for these props are designed to break if you hit bottom and spares are about only $40.
http://www.mercurymarine.com/dealerlocator
and select "demo props" there are a list of dealers that will let you try props before you buy . . . You can always buy a aluminum 19P after trying some other props.