You really have to watch your temps though. Higher toothed pinion is going to raise temps, so like I said, experiment with it and find out what it likes.
4s will definitely make it fast. Also keep in mind, the more juice you pump to her the more parts are going to break and if it was a RTR they...
I don't understand why you would want to take that chance. It really isn't a matter of how much you can overvolt it, it's a matter of, if you overvolt it you will burn it up. If you want the top possible speed you can with your Slash then experiment with your gears. A pinion gear set is a lot...