Interesting stuff. Failing to hit a number could be indicative of a problem. Would need to be > 10% for me to be concerned. Good responses by many folks here. I'll suggest WOT performance is mostly about understanding if your boat (hull, prop or engine) has degraded performance (which'll cost you $ later.)
I specifically added 2" of pitch when I replaced my props and very happy about it. Yes, I did lose a little top speed but couldn't even tell you how much. Why? Because I would never run my engines at that RPM anyway. Across the rest of the operating range, I gained a knot. That's because the available power curve (look up any engine) is substantially linear, a ramp that flattens off at high RPM while the power demand curve (what you need for each speed) takes off exponentially; power demand is essentially cubic in speed & disregarding slip that means RPM. Maybe I should make a sketch but you're probably picturing it: below WOT, the engine is loafing. That can even be "bad" in that you don't run warm enough. This hit me when, running at 1900, I calculated 90HP (total) to move the boat (near hull speed) based on fuel consumption. I was way WAY under rated power: 2 Lehmans rated at 120HP/2500rpm. Of course: it was dialed to hit max rated power at 2500 at speeds where the boat drag would be way higher. At lower speed, power demand falls of exponentially, *way faster than available power.* For instance, 1650RPM instead of 2500RPM? 50% less RPM and boat speed but that's more than a 3x reduction in power. Engines loafing at 40hp each. I wanted more efficiency and therefore to operate closer to rated power in the heart of my speed range (1500-1800). Prop guy argued. Shop guy argued. "You'll never hit WOT." Did it anyway & very happy. A bit overpropped maybe but cruising faster at lower RPM. top of the power curve is flat. You won't hit WOT but very close to wide open Power, so you won't even lose much speed. Just like on the freeway, just like a long bike ride: if you're not running flat out, consider a taller gear.