I had the i5 8400 and now have a i7 10700kf and did not gain much fps because my gpu is also limiting.You may likely get a bigger performance bump due to having a much more powerful gpu.
The i5 8400 is bound to limit you especially with a 3070 and it was limiting my 2060super now with the i7 smoother sim, smoother performance with glass cockpit planes and also I can increase LOD sliders without having those long pauses my 8400 did and it does make some difference if you fly at high altitudes.
I would highly recommend getting an 8core 16thread cpu unlocked.i7 9700k will be decent upgrade but a cpu with multithreading may help you with DX12,
To sum it up I only got an fps increase with airliners and glass cockpit aircraft.