The optimal range of most PSU is 50-70 percent of the rated load.
simply not so. good quality psu usually performs at loads of 90% better than at 50%.
go and see some graphs at
www.jonnyguru.comfyi also psu output is the number on the box, not the consumption at the wall.
Not sure where you got that information, certainly not at jonnyguru. A PSU peak efficiency is at around 50% and it decreases as you get either lower or higher. No PSU will perform better at 90% than it does at 50%. The reason you can see a lot of tests at higher loads in jonnyguru is because thats exactly where most PSUs have difficulty hitting their 80+ ratings.
As for your last statement Im not sure what you mean, but the number on the box is the effective power the PSU can supposedly deliver to your PC components. As no PSU has 100% efficiency, it will draw more than that from the wall to effectively deliver what it says on the box. How much more will depend on its efficiency
PS: Just do a simple google image search for PSU efficiency curve