If your plants absolutely need to be transplanted, you might just have to bite the bullet and get them in a larger pot to allow for more growth. But, they must be monsters to be rootbound in 5gal pots. Have you actually taken a look at the rootball? Are the roots popping out of holes in the bottom of the pot... or are they just more noticeable up top? Repeated waterings can wash away soil from the bottom of the stem and expose the roots, so seeing roots up top doesn't necessarily mean it's rootbound. Make sure there aren't other factors that may be contributing to slower growth before you move to a bigger pot. You obviously don't want to risk stressing the plant if you don't have to.