Putting meth on the car without pulling the fuel will make the car bog badly. I have to take 5-10% off my tune with my ssafc when I run my meth setup. Meth is an unmetered fuel. The ecu doesn't know whats going on, and cant compensate for it.
If your afc is 12:1, and you spray meth/water 50/50, it will drop below 10:1 and you car will bog.
The connecting rods in our motors are like toothpicks, and the pistons like butter. Going lean will send bits of your motor all over the road. I have put 5 engines in my car in the last year. I know what works and what doesn't.