Yes never a good idea to overfill a tank but how much this affects movement depends on the design of the tank. The Jobo has a great deal of expansion available due to the shape of the top but Durst tanks much less so. I have never tried it but I suspect that it would be possible in the case of a Durst tank to fill it right up to the top of the light trapped filling hole and have almost the equivalent of a solid block of developer such that on inversion very little movement of the liquid would take place