My response to the OP is since you are running a very specific spec class, why do you suspect your batteries are holding you back? The only way to really know if they are is if you did benchmark tests when you bought them and have bench-marked them since and have seen a significant drop in performance. At a minimum you could benchmark them now and compare them to whom ever is beating you. You might find that a person finishing in front of you is actually using lesser equipment but has a better setup car and is a better driver.
It is easy to feel like we need to buy our way to a win with newer equipment, but rarely do any of us get the most out of what we have. If your car is dropping off during the race with your current equipment, your setup just may not be efficient. If possible, see if any of the faster guys will let you borrow their battery and see if your results improve.