Hmmm.....the 6400 lb displacement, 27' cruiser I'm considering is powered by
a single 250 hp gas engine. Would you say that it is underpowered?

It depends what kind of performance and engine durability you are
expecting. I have a 5000 lb I/O cuddy with a 5.7L 260 hp engine. At
3400 RPM it cruises at 24 kts in flat water which I regard as OK but
nothing exceptional. It burns 13 gph at that speed and has been
reasonably reliable.

The point of my previous post was that there is an upper weight limit
where it becomes impossible to get adequate performance from gas
engines, even with fuel injected big blocks. At 6400 lbs you are well
under that limit but a bit underpowered unless you go to twins or
bigger engine. Twins become difficult to maintain in a boat that size
because of close engine spacing.