Is there a point where putting more time in a practice category does not yield any better results? For example, I pretty much never put more than 20-25 minutes in any category. Am I missing out on some improvement here? 

I would really like to dump like 50 minutes in a category and get it maxed out quickly, then move on to the next, but my experience tells me this is not an option.

there are diminishing returns. At a certain point you begin to see less improvement for the minutes. Most coaches tend to not go above 25 minutes or so...