But you do know, that radeonsi is catching up on catalyst/fglrx?
Also, the officially by AMD supported driver stack on Linux is supposed to be building upon the current state of the foss drivers*. Catalyst will only reside for professional uses (FirePro). So sooner or later, ue4 should work together with the foss stack, as long at it wants to wear the attribute ‘linux-compatible’. I’d rather not start using the dropped catalyst now.
*plus the new amdgpu kernel module, which is designed for GCN>1.2 cards (currently tonga and fiji based products = 285, 380, Fury (X), Nano) but can also be activated for GCN 1.1. The current radeon module will be on par feature-wise and also be supported by amd for older gpus (GCN 1.0/1.1).