What's the boat like when you bring 3 guests on board? I'd bet you'll see similar results.
This is why I do my prop/performance testing with all our stuff onboard, and my tanks mostly full.
You're previous note indicated that you're at 54-5500 rpm, but were able to get to 5600. Through all my prop testing I've learned that the faster the boat is going, the higher up engine trim must be. I get used to 2-3 bars as that's general cruising. But at WOT, my engine trim is much higher.
On our SE Alaska trip, before we left, I had Autoglide calibrated. It ran my tabs in auto mode just fine. First couple days of the trip, my mpg was extremely poor. I looked at my trim tabs, both were extended at 100%. The boat pitch was like, +3 degrees, it's normally around 0 or +1 degree. We had too much weight in the boat, in the stern, thus, even with the trim tabs fully deployed, the bow couldn't come down to "normal", so autoglide kept trying. I manually adjusted the tabs, then set a "favorites" to override the default calibration and all was well after that.
WInd matters too. I can feel the difference in handling (and see performance suffer) when I point my bow into 15 knots of wind gusts.
Weight easily makes a big difference on handling on our trailerable boats.