Thanks everyone for the info, there's not much commentary out there on how well a triple 5k display iMac Pro setup runs. I bought an 8-core iMac Pro myself this week, as one of our stores in Australia was clearing stock of the 8-core base model for $5999 AUD - almost 18% off the usual $7299 AUD price. I may regret this slightly if Apple update the iMac Pro soon, but there's also a chance they'll drop the iMac Pro altogether.
My experience with this setup so far has been amazing. My previous setup was a 16 inch i9 MacBook Pro 4GB 5500M model (the highest non BTO model) running 2 LG 5K displays. Even though the new 16 inch MBPs are very capable machines, this iMac Pro feels like the first Mac I've owned that can adequately drive 5K displays without constant fan noise and intermittent throttling. Everything is smooth as butter, and in a couple of days use I've never seen the computer struggle at all, and it's whisper-quiet even under load.
What's interesting is that the single core GeekBench scores for the i9 16inch MBP and the 8-core iMac Pro are basically the same, but in general use this setup is just miles better. The extra 16GB RAM, Vega GPU and cooling system also play a big part in making this setup so much smoother.
I'm really impressed, as this thing is driving a LOT of pixels. Makes me hope that Apple don't actually kill the iMac Pro model in general, as the only other machine that can drive 3x 5K displays is a Mac Pro, and then you need to buy an extra 5K display (and you currently can't get the LG 5K anyway).