Go 7600 might be faster than Go 6800. It depends on what they did to make the 7600 mobile. 6800 Go is a 12 pipe chip at 290 MHz default, and the Ultra is also 12 pipe but 450 MHz. 7600 is actually almost the same design as Go 6800. Same number of pixel and vertex shaders, same number of ROPs and texture units. It all depends on the clock speeds. Go 6800 has a 256-bit memory bus though, a lot better than the 128-bit bus of 7600.
7600's architecture is probably more efficient. I bet it's faster than Go 6800, and close to Go 6800 Ultra if overclocked to the same core clock.