2001 Sunseeker '50 w/ Cat 3196 fuel consumption?

Optimal speed for best distance would be around 800 RPMS. What HP are these engines?

A Sunseeker 50 with the 660hp Cat should be the Camargue, correct?
Anyhow, you always get the best range at displacement speed, with ANY planing boat.
But that's very far from being an ideal boat for cruising at 800rpm and 8 or 9 knots for 1100 Nm...!
The most economical planing speed is around 1800/1900 rpm, making 23/25 kts and burning 16/18 gph/engine.
Just do the math...

That's almost 9 and 10 litres per Nm respectively.
Way too much at those speeds, for that boat/engines.
R U sure that everything is in tune, clean hull, etc.?
Btw, she should be capable of much more than 28kts, as Liam already mentioned.
If that's all you could achieve, considering also the fuel burn numbers, it looks like engines are more loaded than they should be.

This is more than our Delta 80 with triple IPS1200 is using and about twice the Delta 54 at the same speeds...

Speed numbers are way off for the rpm and fuel burn, are you using the trim tabs to properly trim the boat. In a sunseeker you can easily gain 2-3 knots by properly trimming the tabs down to the right ride angle? Also I've managed/maintained a 2007 62' Predator since 2011, in 2014 we had bottom paint flaking off and the bottom paint was thick, soda blasting it to gelcoat, putting 6 epoxy barrier coats and 2 coats of bottom paint and we picked up 3 knots, instantly. What RPMS are you making WOT? What load at WOT? What speed at 80% load? Are both props showing even load factors? Any vibration? Boats usually slow down as they get old but 6-7 knots is considerable/alarming.