Memory is the same across all GPUs except for some weirdo models, so +400 is the same for everyone.
The problem is there's probably 50+% of nvidia users out there that mine in P2 mode and just don't know how to force the miner to P0. So +400 will only be something like "+200 from stock" for a lot of folks. 
How do you force 10 series cards into p0 mode?
I believe nvidia inspector has this option.
Hmm afaik that only works for 9 series gpus. But I'm keen to know if there is a way for 10 series.