That is kind of hard to say
There are a lot of factors here to consider.
Although 1500 hours on a marine diesel is not a ton we have to consider how it was maintained during that period of time.
I think you mentioned it was beat up a bit when you got it.
How efficient is the hull of the boat. Moss, barnacles,etc etc.
How big are the wheels you turning? Are they perhaps tad too big and your working the engines too hard. ( everyone wants or wanted more speed so,,,,,,,,, you don't get something for nothing )
A marine engine works harder, much harder than any automotive engine.
A generator running at full load would be about the only thing that may work harder.
How far have you got the throttles pushed to get the 2800 ?
How much smoke ?
White or black
Are the injectors dribbling or leaking
Condition of the fuel pump(s)
How much blow by on the 1500 hr engine compared to the newer one
More smoke ?
Do you have more throttle input on the 1500 engine compared to the newer one to achieve the same results.
Did the spec sheet say what the original prop size of the boat was ?
Back in the 70's the diesel fuel itself produced more power just from how it was made. There have been many changes to diesel fuel formulation over the years the biggest is the reduction in sulpher content.
You are no doubt burning winter diesel right now and perhaps at the beginning of May when they start shipping summer diesel it will pick up a bit.
The new rules in Canada ( 2011 I think ) is that there must be 3% ( IIRC) bio content in diesel.
Because of the colder climates here ( in the rest of Canada not tropical like the west coast ) the bio portion gets cloudy and waxy quicker than just the summer formula. Therefore, the summer fuel will have up to 5% bio in it from May to October which ( so I am told ) balances to 3% +/- over the year.
Winter diesel is much lighter with the addition of more kerosene etc to keep it flowing in the cold and the cetane level drops as well with these additives. There is some debate that the refining process is different and I tend to believe that. In Canada you don't get a choice of diesel fuels like you can in the US. It's just DIESEL fuel. My personal experience with my SD 25 is I pick up approx 5-7 mpgs (imp) in the summer.
Heavier the fuel the hotter the flame generally.
I know long winded to get here but,,,,,,,,,,,,,,,,,,,
Spec sheets printed in the 70's for fuel blended/refined/formulated (whatever ) then will not necessarily equate to what is available now.
Add to that the hours on the engines, how big the props your swinging, the condition of the hull etc an extra gallon per hour may not be all that unreasonable.
IMO that is.
Just as a thought here you may want to look at getting some Cetane booster and try that.
Stanadyne Performance Formula is a quality product and, with the ULSD your using, and the old pumps, an additive such as this is recommended to keep the seals from dying ( which they eventually will ). There are a few other products out there as well. Power Service makes a pretty good one but the fuel injection rebuild places here all send a bottle of Stanadyne home with a freshly serviced pump. I think that Fred Holmes out there does as well.
I use it every tank even after the rebuild.
See here: http://www.stanadyne.com/view.php?id=45