I found a great recipe for key lime cake, but it seems to lack the key lime flavor I am looking for. It has lime zest and juice in it, still a great cake, but it seems not enough lime kick to it.
My daughter who loves lemon cake thought of a great idea a couple years ago to knock the lemon flavor out of the park. I'm sure it would work for lime too: make a lime syrup and pour it into the bottom of the cake after poking it all over with a thin skewer. The more lime you like, the more syrup you use. As a bonus, it also adds some moisture to the cake. Note that you may need to refrigerate the cake after this step (if it lasts more than a few hours).
Without seeing the recipe, it's difficult to know what can be improved. Here are some possibiliies:
Your easiest addition would be to add more lime zest -- it contains much of the flavor, and it won't significantly throw off the moisture or acid balance in the cake.
If you're going to be stacking the cake, instead of using frosting between the layers, you can make a lime curd, and use that to add extra lime flavor -- but be careful how much you spread on ... if you have too much, it'll function as a slip plane. (and the tops of the cake can slide off ... which makes for an interesting talking point, but not typically what you want when the cake layers look like they're throwing themselves off the cake).
If you're not stacking the cake, use a skewer to poke vertical holes all over the cake, and then pour a lime syrup over it, so it can soak into the cake. You may need to let it dry for a couple of hours or overnight before decorating.
If you're going to be decorating the cake, use lime zest and juice in the frosting. I'd actually recommend a royal icing over a whipped frosting. Start with half of the liquid called for as lime juice, mix it, and then add water to get it to the consistency you want. You can also mix in the lime zest if you're just covering the cake and not trying to pipe decorations.
If you're not going to be decorating the cake, make a glaze from lime juice, lime zest and and powdered sugar, and pour that over the cake.
make sure you are getting the most out of your limes with 5 Fabulous Lime Tips
remember Key limes are more tart than Tahitian limes (given the choice...)
if the recipe calls for salt-- reduce the amount (salt increases the sweetness, muting the tart & tang of the limes)
you might add a little concentrated lime juice (just a splash!)
Good luck with the cake :)