steve;
Chief asked me to put in my two cents worth and sold nearly 20-25% of the entire H26 production line.
First the transom is only rated for up to 10 hp. Going with anything larger, you may seriously damage the transom. I would opt for an electric start with long shaft
The 9.9 is all the power you will need and do not let anyone else tell you otherwise.
Even at full power, you will be plowing. This is when the bow of the boat will raise up as it is hitting a wall of water slowing the boat down. Remember this is a displacement hull and not a planning hull. You will have to depower to get the bow down and at that point you have reached max speed under power.
I hope this answers your question. Chief, thank you sir for the heads up.