If there's enough developer to cover the film safely in a spiral tank there will be enough.
There are a small number of exceptions, though, for some extremely dilute developers, (for example Rodinal diluted 1+100), and some unstable developers (like PMK) where having more can prevent the problems of developer weakening too much.
As has been pointed out, the size of the air space in the tank will have a significant effect on the kind of agitation you get. Opinions differ. I think that a large air space is ok as long as you remember that even very gentle inversion will move the liquid much more than in a nearly full tank.