Just made some SysSpeed tests with both a DCE CV64/3D and a Phase5 CV64/3D card. Don't know if SysSpeed is a good benchmarking tool though.
Anyway, the results can be seen
here. With a DCE CV64/3D the rom access is improved a bit, however there are some massive improvements in other areas like Intuition/WinLayer, Intuition/Size, Intuition/Move and all Graphics/Area fields.
The DCE graphics card is only sligthly slower than the Phase5 in a few areas, so it seems like there is a difference in speed between the two cards. Not that big a surprise since the DCE has 50 ns ram chips, while the Phase5 has 70 ns ram chips.