50% load sweet spot? That can't be even remotely true. I agree that having some headroom is good but running 1600w for 800w load is just not a thing.
Why can't that be true? Because you've never heard of it? 50% - 70% is usually the best efficiency for a PSU. Depends on the PSU and curve. 85% or less is recommended for a longer life-cycle. I always aim for ~50% load on PSU understanding that demand usually increases.
http://www.anandtech.com/show/2624/3
Edit: And I currently have an 800W load on a 1600W PSU. So I guess it is a thing.
You are correct with the around 50% sweet spot, but the difference in the efficiency is so negligible that I wouldnt bother with it.
Personally I am fine with 90% load of the PSU.
You also need to remember that 1000 is the wattage PSU can dish out to PC, so 90% load will register as around 990W at the wall.
Every PSU with a certificate should be browseable on this site: (I am actually amazed how this site is not common-knowledge by now)
https://www.plugloadsolutions.com/80PlusPowerSuppliesDetail.aspx?id=25&type=2
This link will take you to corsair PSUs.
Now lets take a look at RM1000i efficiency curve: https://plugloadsolutions.com/psu_reports/CORSAIR_RPS0010%20(CP-9020084)%20(RM1000I)_1000W_ECOS%204336_Report.pdf
As you can see it stays pretty much constant.