It really depends on the class: the teachings in tai chi classes vary widely because there's no official governing body. In my opinion, all they (all) have in common is that the students (mostly) move slowly and look sort of martial.
Now, in a
good class, under a good master, you'd learn balance, sensitivity, power without unnecessary tension, moving and expressing force from your core ... all sorts of good things that could help you anticipate, move with flow and develop a root. Good taijiquan works well when you are in contact with the opponent: touch is important. You'd also learn applications in a good class.
However ... in an
average class, you'd probably just learn some calming choreography.
In a really
bad class, you might injure your knees!
I think you'd improve your
flexibility in a yoga class, starting with gentle Hatha, moving up to Vinyasa. In the latter, you might also get some strength benefits, since you have to hold stuff like leg-up-in-the-air poses for a while. Meditation & centering? It depends on the class.
Would a good taijiquan class make you a better fighter? It depends on where you feel your fighting skills are deficient now, and how you define the idea of a better fighter. But from what I've seen and heard, a good tiajiquan teacher can help you sense an opponent's intentions better (when touching), improvise more (because you're watching/sensing what the opponent is doing rather than sticking to a script), be more sneaky (part of taijiquan is about fooling the opponent) and maybe help you appreciate your own art from a different point of view.
The taijiquan class would probably act as a
complement to your kenpo. You're not
likely to get more sparring in a taijiquan class, so to be a better fighter, I'd imagine you'd need to keep up your sparring in kenpo: "pressure-testing," as it's called.
Xue Sheng could chime in here: while teachings vary widely, there's a general consensus that Chen classes tend to be more martial than Yang classes. But that's a broad generalization. If you have martial needs, ask what style they teach in the class you're investigating. If they can't address that question with confidence, you're in a feel-good-comfy class, probably not for you.
My 2 cents.