Yeah, as long as you're pulling more than like 20% of the rated power then having a slightly overspec power supply is only going to bring you good things.
To my knowledge, PSUs are rated up to a declared power output but unless you have hardware that requires all of its power, the PSU doesn't put out the full charge.
Sure, but that difference is less than the variability based on load. The difference in efficiency between a Gold and Platinum PSU is like 4%, but load can vary that efficiency by 10 or 15%