Tom - you should be able to get 30mph or more out of her. I had a PC 175 with a 40 merc and she ran 28-29 fully loaded with 2 adults and 2 kids, and Steve R. has a '91 Pro 17 with a 50, and I believe when he borrowed my GPS last April he was at 33 or 34mph top end. So its either the Prop, the motor hieght, the motor or a combination of all 3.
Steve - If you read this post, what prop are you running so Tom has a starting point?