There’s no actual limitation on the number of courses that you can take while on the new Post 9/11 GI Bill. There is a cap on the total fees per term (which you mention), but you are allowed to take more than a full-time course load if you can handle it.
There are some practical limitations. The book stipend doesn’t increase over $1,000 per year, so your books may cost more than what is allotted for them. The difference should be manageable, though. Other than that, if you can take extra classes, go ahead. It helps you get the most out of your GI Bill, and your time at the college.