There are a few things.....
1) The stock speed'o SUCKS !!! They are never right. So, the only way to really know your speed is with a GPS. One run, up river, and one pass down... then take the average. (FYI, my XP speedo reads about 10 mph low on the top)
2) You are a little on the heavy side, so that will drag your speed down a little. (3~4 mph)
3) The nujet impellers aren't very good, and we see mixed reports. So, if you find that your speed isn't into the mid 50's... that's the first place I would look.
4) With the nozzle up... you are loosing thrust. Both because of redirecting, and because it's not "In-Line" with the hull. AND... because of that.............
5) Your RPM's are low. If your RPM's don't come up to 7000 with the nozzle flat... then make sure your ignition timing is right... your flame arestor is clean... your throttle body is opening all the way..... and that your water regulator is set properly. If all that checks... I would go back to the impeller, and blame it.