Not worth your time, money or energy with them. My personal opinion is that they dont care whether you pass or fail. They might not even sign you off. My week at CFI school consisted of 2 days of hardcore personal studying(they dont teach anything except how to instruct a Vmc Demo), 2 flights of 2 hours of instruction on the finer points of steep turns in the Seminole, and then a quick 20-30 minute hop for spin training. Spin training consisted of the CFI showing you one to the left and one to the right, then you doing two to the left and two to the right and going home. Kill proofing yourself? no.
Your going to do what your going to do, again just my opinion. But you can get better training locally, where they should be actually invested in you succeeding than spending the money on the airfare, etc to have some kid teach you how to be a CFI, and then some 2 yr CFI is just used to endorse your book and give you spin training. Some things ATP is good for, others they are not, and CFI school is definitely not their specialty.
If anything, some of the "good" cfi responses on here are going to be ATP instructors trying to keep the circle of training going so they still have a job. Giving my opinion on here irritates a couple of folks, enough to have a certain CFI for ATP "attempt" to discredit me. Truth hurts.
One other thing to take note, Do you have a job where you would benefit from being a MEI? the CFI-I is pretty much just like your Instrument checkride.
And since these are addons for you, they are going to be non-event for the most part. I dont know why you wouldn't get the CFI-I locally, besides airplane rental, it'd be only an hour or two of training if that; before I'd assume you'd be ready. The MEI is similar to your PMEL, whats different? There isn't much to that either. Only thing that would suck for the MEI is finding a twin to do the checkride in that isnt going to break the bank.
If you were up to snuff I would probably say 5-7 hrs of multi training, and 3-4 single engine. Most likely less if you were on the ball. Your a CFI, so you should have the knowledge and experience. Would all that cost more than $4k at ATP?
Anyways, wish you the best of luck in whatever you decide.