Pushing a PSU - How Far?

I have heard / read everything from never exceed 60% of the PSU’s rating to 90%.
I know that efficiency goes down as the load increases, with the loss going to heat.

But like a car with a speedometer that goes to 190mph, there’s federal case law (FTC) that says that the car must be capable of that speed and maintain it safely, regardless of the speed limit (think private road). So there’s the rating, and the overhead built in to electronics.

So, how far can you push the PSU? Not going to go over 90%, but wondering about this (certified 80 Plus) PSU. Plus it has a 10 year warranty.

EVGA SuperNOVA 750 G+, 80 Plus Gold 750W, Fully Modular, FDB Fan, 10 Year Warranty.