WMB, as long as the starter is rated for 24 volts then no problem unless you have to keep it spinning for A long time then like mentioned it will over heat. Most of the cheap starters are only rated for 12 volts. Mine is only A 12 volt starter so by going over to the car jumper I'm getting A lot more AMPs and A lot more zing out of it.
One of my friends had to go to 24 volts for his new G-45 and bought the bigger starter. If used too much it got hot too. I like the idea of just 18 volts, never gave it any thought but think I will give it A try next time I need more poop.