Yes, you should see at least 60mph, probably closer to 65 mph. I will assume that this is an in-line 6 cylinder Mercury 1500 outboard as that is most likely the case.
First thing first, what prop are you using? Diameter and Pitch? Stainless or aluminum?
Suggestion....running a tunnel with a jack plate, you need a stainless propeller.
Next, do you have a water pressure gauge. You need one, it is a must. I don't know the in-lines as well as the v-6 motors, but I am going to say you need at least 12 lbs. of water pressure.
Next, if you don't have a nose cone with low water pick up, this will definately help. I would assume that you do have one though if you are running a jack plate.
The prop could be the problem. I couldn't imaging running an aluminum prop near the surface. The vibration would have to be terrible.
If you have done all this, it has to be the motor? Compression check? Carbs?
Good Luck