With 4s you can reach 80km/h with ease btw. (good motor/good cells)
If you go 5S it's more than enough power. 6S is overkill in my opinion. Nice to show off, but useless on the track. 5S is a nice in betweener. with good cells (such as flightpower 5000, or any enerland based pack for that matter) gives you a good 17.5V+@110A load.