The ATI Radeon 5870 that came with my old 2010 handles UE4 just fine. The problem that OP is describing does not sound like a GPU problem to me.
Where are you getting this information? While the FirePros are workstation class GPUs, they actually work very well for games and for UE4. They’re not a great value for that purpose, I’ll admit, but I use UE4 all day long on my 2013 . It’s true that you can get a lot more GPU power for your money with a gaming card, but it’s simply not true that the GPUs are not good for UE4 or for games.
That all being said, if you’re considering a new machine primarily for game dev, you’re going to be able to get a lot more rig for your money if you go Windows. It’s unfortunate, but true. There are just a lot more options when it comes to GPUs for Windows machines. You can get a machine that handles UE4 better than a $3000+ baseline for half the prices or less.
Of course, then you have to put up with Windows.
Either way, I’m pretty certain that the problem is not your graphics card. While the cards that came in those Mac Pros are not as good as the high-end consumer GPUs available today, they are still perfectly capable of driving UE4 on a dual 27" monitor setup. I know, because I’ve done it quite a bit.