The only reason to go with x99 is if you're going 3-way or 4-way sli. The only ROG maximus option would be the maximus vi extreme and it has a plx chip to add the pcie lanes for 3-way 4-way sli, but the plx chip adds a slight latency. X99 with the 5930k or 5960x has the available pcie lanes so no latency.
http://www.kitguru.net/components/cpu/luke-hill/intel-core-i7-5820k-haswell-e-6-core-cpu-review/7/If you went with x99 and the 5960x it would future proof you longer being the 5960x is an 8 core cpu but that is a much more expensive build. The gpu area is where you will notice the performance increase in a gaming pc.
Save yourself a little cash go with a maximus build and add another 980ti for gaming at 4k. You could save a little more cash by going with the hero vii, I'd really like to see you get two 980ti's for 4k gaming. There would be nothing worse than building a 4k gaming pc only to find out you have stuttery gameplay.
Main differences between the formula vii and hero vii
The formula has onboard bluetooth and wifi
10 sata ports instead of 8 on the hero
6 usb 3.0 ports vs. 4 on the hero
The formula has the armor
8 fan headers vs. 6 on the hero
The formula has an 8 pin and a 4 pin cpu power connectors
The formula has the built in water cooling option for the motherboard vrm's
The formula has sata express.
If you get a good 4790k cpu from intel you can overclock it to 4.9GHz - 5.0GHz with an all in one liquid cpu cooler such as the corsair h100i - h110 and the hero vii is capable of that.
https://rog.asus.com/forum/showthread.php?61177-Please-verify-my-OC-settings-and-Tests-vii-Hero-i7-4...I like your choices so far.