I've got a couple questions about SLP loudmouth exhaust on a '00 TA. How much hp gain to you get from the LM? Is it worth the money or would cutouts be better?

Depends on future mods.
Maybe 10 RWHP gain with the cat-back. Which is pretty universal for catbacks.

I do not recommend the Loudmouth if you plan on running longtube headers in the future. Sounds good with stock manifolds, but not once flowing really well. Too much drone, and very raspy.

what do you recommend if i plan to get headers?

Magnaflow.