There are differences between federal regulations and the standards of ABYC.
So let's walk through this a bit:
Engine weight in boatbuilding is used purely for capacity calculations and testing.
In the ABYC Standards, we require a capacity plate on boats less than 26 feet, while the federal regulations stop at less than 20 feet. ABYC reviews our engine weight table annually and compares it to the market, so, yes, we do change with technology, modifying the weights engine builders use to calculate and test capacities. Hence, the change.
Your boat doesn't fall under the federal guidelines, which haven't changed weight-wise since the 1980s.
So Grady-White responded to the weight change and was able to recalculate and test this model due to the change. That said, they also had to test the ability of the boat to handle not only the weight but the power of the outboard.
Could it pass the ABYC performance test?
Apparently it could, so they could re-placard it with the increased capacity and horsepower.
You could ask Grady-White for an updated placard for your boat, but generally, most manufacturers would be reluctant to issue a new placard given that there are so many variables in such a situation. They also may have made engineering changes you don't see resulting in the higher horsepower. Bottom line, should something happen that results in any legal issues (such an accident), and you had a 300-hp engine with a 250-hp placard, your defensible position would be compromised.