Speed isn't really the issue with outboard jets. Being able to quickly get on plane and stay on plane is what'll enable you to run the shallows well. If your under what Outboard Jets says as far as weight goes, then you should be fine. They make the pumps, so they know what they're talking about. Once you get everything squared away, I seriously doubt you'll want to ever go back to a prop for running the river.
How is the weight currently setup on your boat? Is your motor tiller or remote steered? The only issue I can see is if it's a tiller and you also have your gas tank in the back of the boat, you might be a bit stern heavy.