You don’t need lots of voltage to go fast. It’s all about gearing, and the right motor KV (and size) for the job.
Given a “standard board” - to increase speed you do one of these:
- Increase physical gearing (a larger motor pulley or a smaller wheel pulley) - though don’t “over gear” your setup - if you gear too high you’ll heat up the motor and ESC.
- Increase voltage - i.e. go up from 6s to 8s. This is a very easy way to increase speed, Make sure your ESC can take the voltage increase (the motor probably will)
- use bigger wheels (same as increasing gearing)
It is best when choosing motor, esc, battery voltage to try and have your board running at max efficiency at full (or near to full) throttle. So, using one of the many speed calculating spreadsheets on the internet, put in the motor KV, battery voltage, gearing, and wheel size and see what top speed it gives. Then knock a bit off. So if it tells you you will max at 25Mph, it will be around 21 ish in the real world.
Now you can change the motor KV, or the battery voltage, or the gearing to get the max top speed you actually want, and build your board around this.
When doing this, if you want to keep things cheap, don’t go above 6s (cheap batteries and chargers).
If you want to use a VESC then you want to head towards 8s or more as the VESC doesn’t run much above 75Amps, and more voltage = lower amps. But now you need more expensive batteries.
If you climb a lot of hills, keep your gearing and KV down and increase voltage. Saying that, I climb steep hills on 6S with car escs.
I recently built a new board with two motor choices. The calcs told me that the high KV motors would theoritically give me 42 mph - this is WAY more than I want. The second motor, the lower KV one, would give 24mph. The supplier of the motors said “don’t worry, get the faster ones and then don’t use full throttle”.
He’s right, and very wrong too. I would be running a very fast setup at less than half throttle all the time. It’s not right to compare it to a turbo car with spare power (a car engine runs just as efficiently at medium and higher revs) . Because my setup would have been running at less than half throttle nearly all the time, it would be hard work on the ESC, and I’d be running less than optimal gearing. To have all that spare speed at hand, but hardly ever used, is akin to driving everywhere in 5th gear - really inefficient. It would be crap up hills too (lacking in torque as the setup is geared for a speed you hardly ever do).
Gear your setup correctly and everything runs cool and happy. I’ve built a few boards now and a change of 1 tooth on a motor pulley can be the difference between overheating and running cool.
When your friend refers to wasted voltage in esc heat he’s mostly right. At part throttle, an esc has to dump the excess - some of this is heat. Hence it’s best to set your board up to run at your normal cruising speed at a point beyond 60% throttle.
I have two 6s boards, and both are geared to run a top speed of around 24 mph. Throttle is around 50% for cruising and I use around 75% going up steep hills, and only 100% when I am wearing a helmet, and that’s not often (not often max speed!). I’m 48 and coming off over 20mph is reasonably likely to break somthing (on me!). My board was previously geared for 35 mph but I geared it down to keep it cool and avoid a traumatic brain injury! 
