Thanks all, great info. OK so that makes more sense now, its MacOS itself which is a limiting factor so best to use Windows via Bootcamp or Linux boot-USB/OS to maximise the gpu.
Any thoughts on HoneyMiner?
Which iMac do you have exactly? Unless its one of the newer ones I don't think it will be worth it to mine. People almost never mined on a Mac. Hence why there isn't much resources about this.
Bootcamp most likely won't work since it technically still runs off OSX. You need to do a direct linux boot usb. From there your biggest issue however will be finding working graphic drivers, once the drivers are installed the rest should be easy.