Re: top speed ?
Look, you already wrote that your boat is rated for 85 hp max. So, with 100hp, it's overpowered. You can do whatever you want to do, but don't expect a lot of praise for running an overpowered boat.
As for speed in tournaments, it isn't necessarily something that makes for the winning angler. I came in third in a local tournament, running my usual 12' aluminum boat and 6hp outboard. It was the smallest boat, by far, in the tournament, and I caught all my fish within a mile of the launch, while the rest of the 40 boats were zooming around the lake. I didn't win, but I did pretty well.
As Tashasdaddy says, you may be disqualified from some tournaments with a boat that has an outboard that exceeds the boat's rating. Up to you. I'll leave the speed racing to others. I go to the lake to fish, not to race.