How far are you in the application cycle? Are your primary apps in? Have you gotten secondaries yet? The answer to your question is yes, it is late, but there is a fine line between applying too late and getting in despite this because you are a competitive applicant.
If you choose to make the decision to take your September test:
1. Do the best you possibly can.
2. Get your LORs IN to the schools you applied to.
3. Have everything COMPLETE at your schools so the only thing they're waiting on is a score.
4. Figure out ahead of time how to get your score to those schools as soon as humanly possible, preferably faster than the speed of light. If this is not possible, buy a Lamborghini.