Here is my input You need to verify your actual speed and just find a hand held gps or some newer phones have gps on them . Next is rpm find someone with a hand held meter and check it . Granted you are getting frustrated but you will get it figured out ! Now if you are getting the correct RPM while under load on the water I have to say its from the engine crankshaft back and the first thing is the Engine coupler which is mounted in rubber and they will go bad and slip . After that its outdrive issue or prop issue , but you changed the prop . Try not to over think your problem and if you know some one with a larger prop ask them to borrow there spare for half day . Last thing All pm you my phone no. and you can call me , I am on ohio game and fish and there is a very good boat mechanic on there and im certain he would talk to you .