I'll throw my five cents in...
I've been modelling in FormZ for 3 years now, 90% of the time on Mac's but I also have a beast of a PC that I use FormZ on occasionally.
The reason for this, is that I find the user interface and the mouse accuracy on my PC not as tight as on the Mac side. I prefer to work on a Mac any day.
That said, I too have encountered the rendering issue you mention. I use Maxwell Render for all my finished images and a single Mac just hasn't got the chops for it. I first tried the home render network. I now have 5 server class machines linked up to power through renderings. Three are inexpensive used servers with Quad core Xeons. It works, and it allows me to render on other machines wheel continuing to work on my main Mac. The down side, you have numerous installs of Maxwell to maintain and trouble shoot.
If you are selling your images, the current solution I use is by far the most flexible. All of may production work goes to a render farm. This produces renders in time delays my home farm couldn't begin to touch and my costs are very reasonable. The render farm solution also allows you to work where ever your laptop can go where there's wifi... and still deliver top notch results. The down side, it's a fixed cost tacked onto your work. I keep my home render farm active for clients who have more time and wish to pare down their costs.
Lastly, and I'm really happy to hear BlueMonkey report there's lots of chatter about the MacPro's uncertain future, you want to keep your options open in terms of rendering hardware. The GPU renderers like Octane, RedShift, FurryBall and Thea (to an extent) leverage the parallel processing power of Nvidia CUDA cards to produce almost live results depending on the cards you are running. The thing is, Apple really ignored 3D and CAD professionals when they designed this sealed MacPro. AMD cards are of no use for GPU rendering, and THAT could cost you big time in the long run.
I have invested in Octane Render and 3 TitanX cards... The learning curve is not inconsiderable, but the speed of the engine means you can tweak your resulting image live, including on the fly depth of field. I know Maxwell is working on GPU support and IMHO, in a deadline based world, instant production level images is where we are headed... Which puts a knife in my Mac heart.
My planned work around? Model in FormZ on my precious macs (I work primarily on a 15" MBP with a quad core i7 and SSD drives and an Nvidia card, that allows me to launch and work with Octane on my laptop) And then I use my PC render station either as a stand alone or as a slave controlled by Octane on the MBP.
It's not a perfect system, but it's an option that allows me some time to consider the future and keeps my computer experience with my Macs unfettered.
Good luck, and let us know what you discover!