04 f115 down on rpm

[outback32]

I have a yamaha f115 in May it was reaching 5200 rpm with a 21 prop and around 5800 with a 17. Top speed 40 with 2 ppl and fuel. A little while afterwards I had to have the lower unit rebuilt. The mechanic had to go back in and work on the lowera second time.After that time I am now getting 4500 rpm with the 21 and 5200 with the 17 and top speed of 34. Weird part is the motor runs excellent idles great runs smooth all the way to top end. Its not sluggish or choking like starved for fuel. Im really confused on what could be going on. Hoping someone here may have an idea. Thanks