Hello westcat,
Here is how Iray works, as I understand it.
The system will determine how much memory a scene will use when you first press the 'render' button in DAZ.
1. If the amount of memory of the scene is less then the amount of ram on your GPU (assuming you have all the correct setting in the IRAY advanced tab) it will render on your GPU, which is the quick and best way to power through the render.
2. If the amount of memory of the scene is greater then the memory of your GPU it will go to CPU render mode, which even on the best CPU's is just not worth waiting for., Anytime this happens I immediately cancel and look at ways to lower the memory usage.
3. I also have a 1080 ti and it is okay to animate with in the newest version of DAZ. They just upgraded the software, it is faster overall, is more efficient in ram usage in real time while navigating the work space etc. And it has a 'denoiser' which lowers render times considerably., you can find in in the 'Render Settings' tab under 'Filter'.
There are caveats to this though, I am animating my first comic/animation hybrid using the above, but I am mostly using it with simple scenes that use low memory and that look very good at say 75% complete, with the denoiser turned on and with image quality set to medium (medium for me anyway). I have several Stimuli animations already completed with 300 iterations per page, and yes they could be better but they look great and will be in the comic.
Final summary, yes you shoud get the best GPU that you can afford. In fact if possible and if I had the cash I would look at a professional GPU, preferably one with 24 GB. But that is a dream build.