@Schugy
took me a bit but i checked with OpenWrt stock firmware and yes... it is faster. But it still caps at 300 Mbit/s which is about 200 Mbit/s below expected but a great great improvement anyway.
Flow offloading actually has a negative effect now and reduces the downstream to about 200 Mbit/s.
Ping timings also slightly improved.
CPU still not fully saturated but at least i see my little one working hard.
As far as the initial question is concerned i would call that solved and I will contact the developers of the custom firmware
Thx Guys