It won't work with an iMac. You need more GPU power. Read the BMD Config documents.
5K is doable provided you're monitoring in 1080 HD. But Temporal NR will not work. If you're delivering HD, you could always render HD as a first pass, then do a secondary pass with ProRes 444 files and do the NR there as a workaround. But only on a full-size desktop Mac with the right graphics cards.