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.
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%
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.