I'm running high voltage setups since a long time but this guide is a very good basis, good work
In racing I'm running 5S 1400kv on small to medium tracks, and 6S 1400kv on bigger tracks. I like the 1350kv on astro because it has more torque which is great when the grip is here.
I asked Randy Pike for a lower kv for 6S application many times but in his opinion the demand is too low, sadly. A 1200 or 1250kv T8 motor in 6S would be awesome and equivalent to a 1900kv in 4S but with less heat, better runtimes, and smoother motor response.