I disagree that those who go get a MFA are for those who only got a taste of it in undergrad. I actually returned to grad school after working professionally for about three years, and a full course of it as undergrad.
I went to grad school to sharpen my skills, make connections, and become the best damn stage manager I could be. Could I have worked professionally with out my MFA? Damn straight. I was, and making a fairly good living at it. But, this is a business, and I knew enough that I didn't know enough to get from where I was to where I wanted to be, and I knew I didn't have the wicked skills and connections you need to make it in this business. In three years, I could have gone and PA'ed for two seasons, and maybe got an ASM gig at some regional theatre. At the end of my time at grad school, I was stage managing at a MAJOR REGIONAL THEATRE, working on shows and productions with top tier teams. Those three years were spent as an investment in my career.
As I have said many times to go, only you or someone who is very familiar with your skill-set can help you make this decision. Grad school is not for everyone - and someone people who grad school is a nice fit, it may not help them in their career. It's a challenge.
I can say that I did get to open up teaching opportunities - yes, you can teach with out an advance degree, but it is much harder to get hired full time without the degree (many more hoops to jump thru.)
I do agree about getting out of school and working for a couple of years. You won't know what you need out of a grad school until you have gone out and worked. You are going to want a very detailed list of things you want out of grad school before you go shopping around - having a strong idea of what work you want to do. If you want to do musicals, then there are schools that are better suited for that. Love doing new work, there are programs geared towards that. Like dance? Opera? classics? You should know these things. Also, knowing what style you have will help a lot - some schools teach an entirely different style then others.
You don't need a MFA to be a good stage manager, but a MFA can help you be a better stage manager. And in a competitive world, where you are running the business of yourself, you are going to want any and every advantage you can get. (remember, in some ways, we are all competing for the same jobs . . . )
So, to answer you question – is the right path for you? Only you really know that. Are you getting the level of jobs you think you should be getting? Are you working at the level you think you should be working at? Are you satisfied with your work on projects you are hired with? Are you frustrated you career is not moving ahead at the pace you want to? Do you find yourself wishing you were better at a certain aspect of you job? Are there types of shows you are not getting hired for that you want to be hired for? Are you frustrated about you not being union yet?