Detroit 12V71

At a 10 kts hull speed they ll burn about 15/16gph

on plane at 18kts, I ll guess 50-60gph

Pretty much no difference at hull speed where beam impact fuel burn more than LOA. On plane the heavier boat will use a little more fuel

12V71s come in different HP tunes. From 450 to 1000 HP.
The most common is the 650 HP 12V71TIs. At hull speed, Pascals comments are correct. After hull speed, MPG is not in normal vocabulary.
We have these TIs in our Bertram. At slow speed, were close to 2 gallons per mile.
Making approx 200 HP.
On plane, 18 knots, approx 50 gph, fast cruise 65 gph, on the pins, near 75 gph.
We don't own our own diesel refinery, so don't go fast often. Usally pour the fuel in navigating a uncooperative inlet.

Find the HP tune and I'll post a RPM/GPH chart for you tonight when I get home.

My perspective here...engine horsepower is engine horsepower, requiring the same amount of fuel to produce the same amount of thrust. If a 12-71 TI at 650 HP or a 12-71 TA at 750 HP is used on each boat (or some other 12-71 variant), they'll both burn the same at the same RPM, only difference being the performance of the hull with respect to speed.

The question then becomes, how comfortable are you at the given speed options for your hull with respect to the quality of the ride, the time required to make the distance, the added cost and maintenance of running harder? That's really then up to you at that point.

That's the high end of the available HP from a 12v71TA. I think the DDEC version was 900+.

Principle still applies. You'll be able to lope at 1400 RPM and make hull speed, burning around 15 per side. I do it at 68 feet and 750 HP 12's, and I burn 25 gallons per hour at 11 knots. In the right weather conditions I'll pull back on one side at a time, back to a fast idle and run 1425/850, and I can cut my 25 back to 21 without losing speed. You'd be slightly higher than these numbers given the extra engine displacement.

Meh...get a good mechanical surveyor aboard, take a close look at hoses, fittings, nooks where oil didn't quite get wiped up, and run your oil samples. The engines are solid if maintained. Don't be scared of old iron. But try to determine whether or not they've been abused. The seller will change the oil for you, so run your oil samples after you sea trial. Sea trial early in your due diligence process.

If they both burn the same amount at the same RPM, how does the TA get the extra horsepower?

Misinterpreting, perhaps poorly written. If you take a 750 HP engine out of a 61 footer and drop it into a 67 footer, you're going to get the same fuel burn at the same RPM. I wasn't comparing fuel burn from a 650 HP to a 750 HP. HP is fuel.

That's each engine. Again, I run twin 12-71 TA's at 750 HP, and at hull speed with the genny running I'm burning 25 gallons per hour combined. That's 12 per side on the mains at 1400 RPM. For 870 HP, you'll be at 15 per engine at the same RPM, or 30 per hour combined.

Detroit Tune;
Same engine block, just different tune parts (injectors, turbo chargers, air coolers) make different hp levels.
Those would be 12V71TAs with ZF clutches.
900 SAE HP, 870 shaft HP.

Stepped in What?
Propeller load on a 650 & 750HP engines will be different, HP per RPM will be different.
This does not take into account of the TD v ZF clutches and probable different ratios either.

You must be running above hull speed. I ve never burned that much at hull speed with any boat i ve run. Ex. 18 GPH with a 70 Johnson with 1400 3412Es… 22 GPH 84 lazzara with 1650 C32… heck the 110 lazzara with 2000hp MTU 16v2000 I run now only burns 25 GPH at 12 1/4 kts