It very well could be. One of the biggest differences between auto and boat engines is how infrequently fuel is a problem in auto engines, and how frequently it's a problem in boat engines.

I think if it was a debris clog, the rough running wouldn't be so RPM dependent, only bogging down >1500, it would affect performance across the whole powerband even if irregularly so.

If there is a chance that the original fuel was 10% ethanol and/or very old by the time you added that 30 gallons and SeaFoam, that could explain the problems you're having.

It doesn't sound like there is a fuel-water separator, because that is usually an easy and obvious thing to check. But water in the fuel would be something to consider.

Age of the fuel is another. The octane rating drops substantially during prolonged storage. Mixing in new gas would help. But maybe they had some shitty 87 octane in there, and it sat for awhile. If the octane rating below is the rated min, the motor can usually can still run, because the ECM compensates for the lower octane by retarding the timing but this is less effective as the RPM get higher, in your case dropping off around 1500.

Even though it is a huge pain in the ass, empty that old stuff and run it through your cars. One tank of ethanol free premium fuel with stabilizer will quickly confirm whether or not it was a fuel issue.