Yeah, it cant be that much different than setting it up in a rustler. I'd disable your punch control, double check your timing is disabled or set to lowest, and set your start power to high. I'm sure the 5700 is fine on it, maybe cut a hole in your body to get some airflow to the esc. Really you shouldn't have to gear that low. Maybe if you're running it through grass, but on hard dirt or pavement 17 or 18 should be more than low enough you'd think. I personally don't have a slash so I can't say. But it seems to me that the drivetrain is the same as a rustler and the weight shouldn't be more than a pound or two difference, there is no way a 5700 is not motor enough to push that around at a higher gear. Especially if the exalted titan wasn't overheating at stock gearing. Play with the castle link software settings, make sure you have the most recent firmware release and maybe borrow a 2s lipo from someone and see if you can nail this thing. Also mounting your esc behind your rx box probably isn't a good place for it, should keep those two as far apart as you can. Also check motor plugs, if there is any corrosion looking spots on any of them, clip them off and direct solder.