Did you just put the nose cone on? is it straight, It may be causing it to cavitate and with power steering you may not feel the steering torque.
just a thought.

Just a story to tell.......back in my old days(???) I bought a Merc Mirage prop to go on my Merc 200 ob..... ran like crap.... just terrible. Had two mechanics & shops look at it.... then someone said- The thru hub exhaust is too small. changed to a SMALLER hub prop and it ran great. The larger hub was stopping up the thru hub exhaust..... the smaller hub allowed gases to exit over & thru the hub. It worked good....
Is the hub the correct one for your drive? Plugged?? Otherwise, I too would look at the slipping hub.

To verify if the hub is slippping take prop off. On the back of the prop take a center punch & punch a dot on the center part & the outer part next to the inter one. Now after you come back from the water check & see if both dots are still lined up. If not, you are spinning your hub. If they are lined up then go back to your prop guy. I have had more props that don't work correctly after a "Prop Shop" worked on them after I took them in for minor ding repairs.

Something is just wrong. Even if you are over propped a little you should be doing more than 40mph. Try to borrow a good 21 mirage plus and see what happens. If you don't pick up at least 10-12 mph then you probably have an engine problem or some serious hull drag. Is the hull clean? I think a 21 mirage plus is the prop you should have for your boat-motor combination. I have a 260 Baja which is actually a 24 footer with a built in swim platform. With the stock 7.4L and a 21 mirage plus it would do 54-55 mph. Your boat should be pretty close to that if everything is working as it should. Good luck.

Thanks guys, I will try all that. Believe it or not, by looking and the prop wash i was wondering if the exhaust could be exiting around the prop instead of though. But i didn't know if that was possible. I have a swim platform on the back so i really can't see much. I think i need to put on a different prop and see what happens. The nose cone was put on a by a dirt bag shop in homestead.... After he had the drive for 3 weeks, I agreed to take the lower unit with the nose cone just to get the drive back. So i really didn't know anything about them.

Thanks again

Gene