So, it works!
Just did clean install Win7 SP1 + AMD 2020 drivers.
3DMark Fire Strike gives 9263 graphics score, 7229 overall (Physics 6069, Combined 3063) - all the data with 4GB RAM.
But there is GPU core frequency jumping, from 870MHz to 1020Mhz.
Is it normal for R9 280X? No way to deep into...