I'm not much of a photographer but I can chip this much in; when I was
asking for advice here some time ago, I knew exactly in what way my gear was limiting me; I could have posted (blurred) photos of my daughter's dance shows and say 'here is where f5.6 and ISO800 just don't cut it'. Once upgraded, I still get (a lot of) bad shots, but now I blame the choice of parameters or the way I hold the camera, or the way I used the flash... better gear would make a difference, but I'm still far from the point where I could confidently say that I gave my best. And I know that because of all those good shots that happen too.
So you have to ask yourself:
1) are there situations where, no matter what, your gear (or APS-C compared to FF) can't pull it off?
2) are they really important to you?
3) more important than the money difference compared to cheaper system?
If answer to all those is Yes then go for it.