Can Music Theory improve your songwriting?

In my experience, theory helps - but with the editing, not the writing. When you're composing something, you want to follow your muse, and just get your ideas down.

Think of it like designing a car or a building - you start with the big picture, the image of the finished product. Once you've got the shape of it down, and only then, do you worry about how many bolts you need, or where the electric sockets should go.

Theory is invaluable in speeding up the editing process. Once you have a first draft of something, you look for weak spots. Knowing theory can suggest alternative approaches for those points, and it'll be a lot faster than solving the problems by experimentation.

But if you write the first draft from theory, rather than from a vision of what it should sound like, the results are going to sound kind of "canned".

I basically agree, about 80 -20. Being creative means using your ear and your sense of rhythm, but music theory is not inseparable from this. Any music theory you know is reflected in your playing, which is ultimately reflected in your song writing. If you look at my "orchestral-indie" songs (as I like to call them), you will see that they are fresh and interesting, because I primarily use my ear in the development of the song.