It seems to be different on different Mac's. There are a heap of users using ElCapitan with v1.15 and it works perfectly.
I have a late 2013 MacRetina 13" i7 and there are no issues with v1.15 and a full 512 channels on all 4 DMX outputs. However we also have an i7 MiniMac and it does have issues above channel 400 on universes 3+4. We also have an ancient MiniMac with a CoreDuo and it struggles with even one universe.
Not sure why, really wish we knew of that Apple could help, but maybe it is different chipsets or drivers, or just which way the wind is blowing.
If all else fails, and it pains me greatly to say it, a low cost Art-Net to DMX box will solve the issue.