I have 3 x 30" monitors (2560x1600 each) and run at 400dpi. It's perfectly usable.
No it's not..
whenever you need to move that far with your hand, ESPECIALLY when seated, there has to be some twisting of the wrist..
All twists of the wrist is bad..
You're expending extra effort and energy for no purpose..
In terms of desktop use, ideally 1600dpi is good, because it dramatically minimizes the amount your hand has to move.. thus more ergonomic..
for gaming, dpi has nothing to do with anything, because ultimately, when you input, that number gets scaled..
So even with pixel precision on a 2D plane, you're getting rounding on the 360* axis.. so if you want to say, GAMING is ok on 400dpi, there would be no objection to that.