Manually shifting an automatic is a very bad idea. Your transmission will fail prematurely if you do that. Redlining an automatic tranny will cause the fluid in the torque converter to heat up to dangerous levels and eventually distroy the tranny.

You won't get much out of bolt-ons from an automatic LS motor. Mostly, it will just be a psychosomatic increase in power because of the extra noise that is produced. You would be lucky to pick up 5 hp out of all that.

Remember, noise does not equal power.