So an update from this weekend:
I installed a brand new stock finish Mirage 19p prop. In slight chop I was able to hit 51mph at 4600 rpm’s. I was also able to cruise at 32 mph at 3000 rpm. This was all with full fuel and 2 people. I ensured that the throttle plate was visually opening all the way. I don't have the capability to scan anything to see if the TPS was showing WOT. Any more thoughts on my low WOT?

Just to circle back a minute...are you going by the speedometer or by a GPS or radar reading?

51 @4600 is around 8% slip, which is much better. Actually very good.

I’m running a Livorsi GPS Speedo in place of the old pitot style. Why would running a 21p cause such high slip numbers?

I wonder if it is possible you have a spun hub on the 21" prop?
You may just be running out of ponies, frankly. Small-block torque is not the same as big block. Still...it just really seems to me that your engine is getting held back, I know some folk with that (albeit later version) engine and it screams in those applications.
One of the problems is you have no idea what the boat ran like before you bought it...and it would be a crap-shoot if the previous owner would tell you the truth. Something definitely seems amiss. A sub-50 mph Outlaw is just unheard of, frankly. How 'wet' are you running? Do you have or can you get pictures of the boat running at full speed? Even at around 50 mph you shouldn't have more than 6' - 8' of the hull in the water. Make sure your tabs are fully retracted. Keep playing with that trim until you blow the prop out, then tuck back in slightly.
Beg, borrow, or steal a scan tool or bite the bullet and have a COMPETENT and qualified Mercruiser tech hook you up or ideally take a ride. I'd be curious about fuel flow, injector duty, etc. Is your fuel tank vent open and working well?
You really do need to determine that your mill is putting out full power. If it is, then you can start looking at things like the X-dimension on the outdrive, or if the hull has any deformities in it. If it isn't making full power, you need to understand why. Or you can just go enjoy the boat for the rest of the season, don't go challenging anyone, lol...and fool with the thing in the off season.

Here’s the latest on my low WOT saga. I purchased Diacom for my laptop and rigged up a fuel pressure gauge. First off, the engine has 370hrs with many of them under 3500 rpm’s. There were some non active codes stored in the history from about 10-15 hours ago (this was prior to me owning the boat). The codes were for an over speed and a couple sensors.

When I ran the boat on the trailer with muffs, I was getting 33-34 psi of fuel pressure (taken from the rail where the schraeder valve is, near the flame arrestor). The spec for this model of 6.2l is 38-43 psi. Any thoughts on this?

Today I brought the laptop out with us and had the wife drive so I could monitor things with Diacom. At WOT the TPS was only measuring 93%. Is this enough, or does the TPS need to be adjusted? Also the tach is off by about 100 rpm at WOT, so it was actually only reving 4450-4500. The spark advance at WOT was 25 degrees.

25° at WOT is pretty low. Even good chambers that burn quick want about 29/30°.

You're heating up your exh valve and engine a bit, while losing power.

Does your TPS give you a voltage as well? What does it read when it's closed.