As a result I've a very low HW-Error rate and devices already start with "best" option.
I have very few HW Errors running @800MHZ and rejection rate is ok (it is between 3-5% mostly)

But when i switch @850MHZ then omg first errors within some minutes

Also found out that my Gridseeds (scrypt only) running much cooler @800MHZ, maybe i should just leave it there.