what fuel pump are you running? just went thru this.. mark

I replaced that last year, electronic stock one from bam marine.

Do you keep the boat in the water, or on a lift, or on a trailer?

Trailer, bottom of hull is spotless.

I'm still stuck, I don't know what to do next!

The heat kills horsepower big difference now from that cool air in the spring or earlier. Have you pulled plug wires one at at a time while running? Bad wire, plug or crossed.
The heat will knock you speed down in Dallas close to what you are seeing 63 to 57mph. Check for stow-a-way strippers from party cove, could be extra weight in the cabin. Several will ad up!

That was the first thing I checked, the wires that is, I really have no experience to base an educated guess, so I don't know where else to check.

I purposely stay away from party cove just for that reason, having strippers on my boat is gonna cause a LOT more problems than losing mphs!

Did you run the engine at idle and pull one wire off at a time and listen for an engine change? Trying to be sure you understand that test.
I see you have NEW plugs Did you use NGK's they work best with high energy ignitions (according to Mercruiser)

Important to Check the cabin! You Close one eye anywhere on that lake and Strippers will move in fast!

Also,
How about your battery? Is it a good high Amp battery? Really effects the ignition.
My guess ignition plugs wires or battery amps. Along with 110 degree weather.
Non NGK's will appear good but break down with this engine. No miss just won't run up with a load (in gear).
Good Luck!

I purchased a new Hydromotive 23p QIV in April, I must have been smoking a LOT of crack that day, because I swear I hit the rev limiter with the new prop. But that is not the case. My whole problem had nothing to do with my engine, it was simply going from a 23p Mirage Plus 3 blade to the 4 blade Hydro. The loss of the MPH was due to the outside temperature of 105 degrees and the lake temp of 96 degrees. Friday I purchased a used 23P Mirage Plus 3 blade, I took her out today, 75 degrees, 77 water temp, bam, 5000 rpm, 62mph.

I did learn that the Mirage Plus 23p is not a true 23, but more like a 21 3/4p and I should went with a 22p Hydro since they are true to size. I did learn quite a bit about my engine, specifically it's running perfectly.