I am not officially payed from my time (this should hopefully change in a few months) but I treat my time as if I was, any time wasted is simply lost profit. I use a high end machine running enthusiast level parts for my personal projects because of this. I don't like to waste time waiting for compilation, rendering or any other performance intensive tasks. For example if I spend 30 minutes a day 5 days a week rendering then that is 2.5 hours every week that I waste. After a year that is 130 hours of productivity lost, if I employed an artists at for $40 (Not actually sure how much an artist is worth) an hour that is $5200 a year for some one to sit on their behind and do nothing. Lets say that I spent an additional $300 on the system and that cut the rendering time in half that saved me $2300 for the first year then $2600 each subsequent year If I was paying someone (or my own personal time.)
From that alone it makes much more sense to buy a system that has a more performance than is necessary. You also have to deal with poorly optimized builds of your game during development so you will need more performance than what your target market would have.
My current gaming/workstation rig is what I consider as reasonable for for game development. (I may be a bit biased on this as I am a PC hardware enthusiast as well.)
Intel i7-5820k 6C/12T
Corsair 32GB DDR4 2666MHz RAM
Nvidia GTX 980 GPU
4x 480GB Intel 730 Series SSDs in RAID 10 for fast redundant storage for OS as well as workstation applications such as Visual Studio, Blender, Perforce client, ect.
1x 3TB Seagate HDD bulk storage. (I never put any of my code or assets on here as thats what my NAS is for).
10Gb Intel X540 T2 NIC with a direct connection to same NIC in my Server.
i was surprised to find the 1080 wasn't that much faster with today's games - then again, they aren't designed for it.
The GTX 1080 is probably one of the biggest leaps in performance that I have seen for GPU's. This is especially true when you factor in cost. I payed nearly the same as the launch price of the GTX 1080 for my GTX 980....the 1080 more or less doubles the performance of my card. It has around 20% more performance than the Titan X at $400 cheaper(at board partner price). They still have to release the Pascal Titan and 1080 ti (assuming that they make those cards but I think its very probable if they stick with their previous trends.) These cards also run much cooler and use less power than the last generation so the possibility of overclocking is much greater as well. This was demonstrated at their launch event with a core clock of 2.1GHz. To put that in perspective my card is OC'd to just over 1500Mhz.
TL;DR GTX 1080 kicks the sh*t out of 9xx series GPU's.