there is also a better way to drill that hole. you dont.

use a greenlee hole punch for the firewall hole, they are available in MANY MANY sizes, usually electricians have these tools (for punching holes in gangboxes or steel studs)

comes with 2 dies, a smaller and larger die, and a large threaded stud and nut.
simply wrench the nut down and you get a perfect, sharp clean hole. no warping, no damage. and all in like 15 seconds.