This is unfortunately so incredibly true. It's really starting to pain me. There is simply no Mac out there with a decent GPU short of buying a 6 year old Mac Pro and then buying a custom flashed GPU for it. Drivers have not been updated AT ALL in 2 years; performance with NVIDIA GPU's improves about 20% if you install NVIDIA's drivers.
This situation is completely ridiculous. I've been on this forum for a while saying that I want a Mac with a decent GPU, especially in the laptop space, and I just simply can't get it. I just cannot get it at all. Even the top end MacBook Pro is underpowered compared to the screen resolution.
The choice of GPU is also often ridiculous. Top of the line 5K iMac just got updated. Shortly before, NVIDIA had released the GTX 980 (without an M!) for mobile platforms. This would literally have been the PERFECT chip to slap into a 5K iMac and get a really competent machine that could run games really nicely at 1440p or 4K and have amazing resolution everywhere else.
But no, they pick AMD's vastly inferior solution that cost the same. What the actual ****. I'm so sick of it. I love OS X and all the great desktop features you get, but the hardware is total junk at this point on the GPU side, and the default drivers are disparagingly bad (although this can be fixed on the NVIDIA side, as others have mentioned).
The software side of this issue is actually relatively small. It's the hardware that sucks more than ever before, and that's why it doesn't make sense for Blizzard to support OS X. There are so few Macs that can run the game at all, and of those that can, virtually none of them can run it above minimum settings and they definitely cannot run it at native resolution.