Exactly.... Mars/Motenergy markets their 0708 and 0709 as Etek motors..... I had no clue which you're referring to. If you're truly speaking of the original B&S Etek, then yes, 48V is probably about it for that motor. I've not seen a curve for 72V on that (nor at 48V for that matter).
If you need 72V, the Perm and 0709 from Motenergy are rated for 72V operation, and as you mentioned, that's dependant on temperature and current as well. The even better solution is an Agni 95R Reinforced motor, they're made for higher RPM... and overvolting