I triede that, on the eu server. But my rejected increased from 0.1% to 2% So im just using the one without country suffix.
If anything, a locked difficulty should drop reject %, not increase it. No country prefix is exactly the same as useast (it proxys to useast actually), would be better for you to just set to useast if thats your closest server.
The closest server to me is the EU one, and i did set a prefix diff to d=128 and that got me higher rejects.