Sea trialed new boat and fuel burn was way off.

At WOT did it reach rated RPM?

Well, they projected 0.15, 0.26 and probably something like 0.50 nmpg.

You got 0.40, 0.26, and 0.16 nmpg.

At 1200, 1500 and 1800 rpm, I would expect 0.55, 0.36 and 0.25 nmpg.

So, I'd say it's definitely underperforming, but no idea why. Could be hull condition or many others things. Was there any vibration?

My first thought for the possible culprit would be either hull or u/w gear fouled - or more likely both.
Reaching 2200 rpm without trying hard, with engines rated for 2300 max, doesn't sound too bad, but there's still one hundred to go.
And interestingly, if you take the two more comparable numbers (i.e. 1840 and 1900 rpm), you got not only a higher fuel burn, but also a slightly higher speed at the lower rpm, which is weird.
I definitely would check that the bottom is clean, before doing another seatrial.

My first thought is whomever wrote the listing got the numbers wrong.

That's a slow boat and may even be overrated in the ad. I'd say 21 knots is in a dream situation and perfect conditions. I'd guess under normal conditions 18 or 19 knots might be tops. I don't understand boats like that. It's not a trawler, not FD, but it's not got the power to plane decently. Whether the hull isn't capable of more or they're just all underpowered I don't know, but there's one in Seattle with MAN 1200's that does 18 knots and cruises at 14. The one in Halifax with CAT 1550's only claims 20 knots. There's a 2007 with CAT 1550's that only claims Max of 18 knots. That's the one that was at the Palm Beach Show. Is that the one you're looking at?

I'm sticking to WOT at 18 knots and lousy performance due to design and hull conditions. It's a rather stereotypical Taiwan Tub. It is designed and built to perform poorly.

The listing fuel burn numbers are wrong. At 1200 rpms you'd be at about 45 GPH combined. 80% load is 112 gph both if propped to achieve WOT. Boat is a typical Taiwanese tub. If a glass was shaking on the FB of a 90' and it was relatively calm, you've got a problem. It's a typical Taiwanese Tub and an off brand one at that.....you truly get exactly what you pay for.

Actually the Yachtworld listing shows a top speed of 18 knots.

To answer both questions.

I've always seen C30s rated at 1550 HP.

I've never seen whisper wall falling down, especially on any Hatteras 80' as in ever. Hatteras 80' is a very high quality boat, compared to what you're looking at.

Hull speed is determined mostly by length, and slightly by hull design.