I got 780 for the Math 2 Subject Test, and I really have no idea if that is considered good or not. Is it really worth it to go through studying and taking the test all over again? My goal is to at least get into UMD at College Park. If possible maybe even higher such as Johns Hopkins. Are subject tests really that important on whether or not I can get into a certain college?
A 780 is impressive! Most colleges are happy with at least a 700+, and as far as I’ve heard, SAT subject tests aren’t that important for being accepted into a good college. Good job! I would keep the 780. 
Agreed. There is no reason to retake a 780. great job!
While 780 is sure a great score, I understand you wish it was 800. If your practice tests were mostly higher than 780, you probably should try to take the test again because you didn’t do as well as you should have. If 780 is higher than most of your practice tests, I think it is logical to accept 780. After all, other things are very important, too. Good luck!