I agree with Drop Top Pony. No CAI needed to install TB/plenum. But that could be your next mod. You are doing all the right things to get your Stang to breathe better. You've opened up the exhaust (exhale) with the H-pipe and catback. Headers would be a good mod for that too.
With the TB/plenum, you open up the intake (inhale). CAI would help that too. Do your homework on CAI's though. Some are not worth it.
I agree with Drop Top Pony. No CAI needed to install TB/plenum. But that could be your next mod. You are doing all the right things to get your Stang to breathe better. You've opened up the exhaust (exhale) with the H-pipe and catback. Headers would be a good mod for that too.
With the TB/plenum, you open up the intake (inhale). CAI would help that too. Do your homework on CAI's though. Some are not worth it.
I know you already have 3.73's, but I would really think about swapping to 4.10's. If you are going to go wtih plenum/TB, I would go with BBK. I know accufab is considered the best, but to me, it's just not worth the extra money. Headers are a good mod too, but if you do it, go LT's.
I suggest some lower control arms as your next mod. Also, inspect your front and rear suspension and replace some of the crappy stock rubber bushings with polyurethane, delrin, or even aluminum ones. Check/lube/replace the steering rack bushings, a-arm bushings, swaybar bushings, sway bar end links, ball joint play, tie rod play, tie rod boot condition. Check it all while you are down there and replace parts as necessary. If it is a stock rubber bushing or shock it is most likely shot.
Just remember the more hard core you make your suspension the more harsh and noisy the ride. Also, if you lower improperly you can easily spin out in loose gravel or puddles. So make sure you do your lowering properly using all necessary parts for the amount you lower. The lower you go the higher the cost and amount of parts needed.