Are you talking about voltage input to the Rx -> Servo and amount of torque supplied by the servo where 6V is typically less torque than running on 7.2V?

There are many brands of ESC's which offer 7V BEC's so I don't think HV is limited to nitro.

When it comes to 1/10 buggy classes, I try to select a servo that's rated 150oz-in+
for 1/10 SCT/ST classes I'll target 200oz-in+
for 1/8 buggy 250oz-in+
for 1/8 truggy 300oz-in+

I've yet to find the need to run HV on any of my servos even though most of them are HV compatible.