I was about to recommend to the
AMD Hardware hack to a 580, but export options are limited. I have executed this on my cMP. To to advantage of this after performing the necessary steps, you choose the x264 (VideoToolBox) output option. Comes close to doubling the export speed.
But I can't see if it works on iMovie, because I have no way of comparing before and after.
FWIW, I exported a 9-minute GH5 HD clip, and according to Activity Monitor, the RX580 was fully utilized, while the CPU threads never reached 50%. Took about 2:15 minutes to complete (Quality:High; Compress:Faster)