I have now tried the glTexSubImage3D call on ATI hardware instead of Nvidia. It seems the performance of glTexSubImage3D is considerably better on ATI than on Nvidia.

For a 2x2x2 texture, the...

Yes, the increase in time is small compared to the increase in texture size. But anyway I was suprised that there was an increase at all. As far as I can see, the overhead should be the same for each...

Greetings,
I use glTexSubImage3D to update single voxels of a 3D texture, that is:

glTexSubImage3D(GL_TEXTURE_3D, 0,
x, y, z, //position
1, 1, 1, //w, h, d
...