I am not as savvy nor have I experimented enough as to be able to answer all your questions, but anyway, I have the same computer as you have: RiMac, i7, R9 M295X, 3 TB Fusion Drive. The drive performance doesn't make it slow by any means, the initial load time if on the longer side, but it's fine otherwise.
Also, I don't think that CrossFire applies to iMacs. It's a technology for dual GPUs, i.e. Mac Pros. The Retina iMac only has a single GPU.
I use the latest Boot Camp drivers from AMD (14.301). The latest generic ones optimized for GTA V have been reported to work with a workaround, but I had nothing but problems and BSODs with them. Be careful.
I run the game at almost full details and it runs great.
I'm attaching a video where you can see all my graphics settings as well as a bit of what the gameplay looks like. Please note that I shot the video during light rainfall in-game, which is when the game performs the worst! There are other times, especially at night, when the game performs buttery smooth.
If you scale down at least some of the settings, you can easily get 60 FPS out of it. It's just that I'm not too much sensitive to lower framerates and prefer high details. It's a matter of personal preference. This is the video I took for you:
https://www.dropbox.com/s/ongvm35ec5xs0gv/video 16-04-15 16 17 04.mov?dl=1