Thanks everyone. Sounds like I need a bigger motor.. prob should have gotten a 15hp not a 9.9hp. Think I’ll loose much on reselling a “used 3 times” 9.9hp mercury 15”?
If you want more HP, look at the Engine specs very carefully. if there is a 9.9 and 15 hp that share the same displacement, your are not going to gain very much, if anything. The 9.9 very likely underrated(that way they can get under HP Limits and Registration of Boats in some Areas. The 15 would get it's extra power by extending the RPM range and moving the Torque Curve to Higher RPMs, with a corresponding decrease in Low End Torque. On a Very Light hull, that isn't a big Deal, but you are having problems getting on Plane, and you need more Low end Grunt. An engine with More Displacement and Higher HP is what you need.
As for your Original Question...
SS props being stronger can be made thinner which raises their Efficiency,it also allows for better Blade designs that enable changes to Engine Height(really not required until speeds over 45 mph). and higher Trim Angles which lessen wetted Area on a Hull. Some Props can literally lift the Entire Hull Vertically, others are designed to keep the Bow Down and were developed to keep Tunnel Hulls from flipping over backwards at high speed
The much commented 'Blade Flex' was marketing Genius, as it has sold Many SS Props. Get an AL Prop,clamp a blade in a Vice, and you will see how Inflexible(about 1/100" on a 20 " prop) it is, besides, it is pushing a Fluid, and the Fluid will yield long before the Blade does. .