I found the problem with my example. The main problem with the timing is, that per default the nvidia driver enables threading optimizations. This defers the execution of some calls. In my example the GenerateMipmap call takes very long for the volume textures and it seems it is deferred until the next TexStorage call. After disabling the threading optimizations GenerateMipmap shows the expected longer execution times. I think i can work around this by building the mipmaps myself in a real world application (this beeing only a test).
Then again THANKS Piers for the great efforts bringing us beta drivers after fixing issues. I think this is something that nvidia should keep up, always having some OpenGL developer drivers available after some serious fixes or additions of new features.
Edit: Any news on the planned availability of GL_ARB_cl_event and cl_khr_gl_event extensions?