High Sierra - RX480 + Final Cut h.264 issue

[pulpinex]

Hi All,
I upgraded to HS few weeks ago.
All seems ok (except sleep/wake)... but first time I used FCP (or Imovie) ... voilà... surprise!
FCP seems work correctly... but when I export my "movie"... appear the trouble!
All right if I export in native Apple Pro res codec...
Error if I decide exporting in H264 codec.

My system: Asus Rampage IV Black Edition, 64GB Ram, Samsung 860 Pro (with pci express adapter) etc... XFX RX 480 8GB.
OSX: 10.13 (same with 10.13.1 beta 17B35a)
FCP: 10.3.4

Tonight (Italy) I will upload also my config.plist and other info.

Someone had same issue? I read about same issue with VEGA 54... (click here)
What about other AMD RX cards?
I'll try the way suggested for the Vega 54... I'll keep you informed.
Thx

 

[pulpinex]

Solved using this suggestion.
But speed step dont work as it should do.

... why?