A summary from many posts I've read in the electric 1/8 conversion thread...... (a lot of valuable information can be found in that thread)

power = voltage x current

With 6S, you are increasing the voltage, so you would run a lower KV motor (1300-1400kv) which will draw less current and produce less heat. A lot of people are getting longer runtimes and lower motor temps by increasing voltage.

If you were to run 6S on a 2200-2600KV, you would have ridiculous power and equally ridiculous motor temps! Not at all a combination suited to racing!

I've been running 4S with a 2550kv motor but I am about to go to 6S with a 1300-1400kv motor to further reduce motor temps and perhaps even see longer runtimes.

In terms of ROAR regulations though, I believe (don't quote me though) that 4S seems to be the 'agreed' upon standard. Doesn't affect local club races in Australia though, but may be an issue for larger events in the US.