When you are under low light situation, I believe there isn't much UV ray that you want to filter, removing the UV filter will increase the light into the camera sensor. I have done the experiment before, by fixing the appeture, and let the camera auto set the speed, I found that by removing the UV filter, the speed increase and when put back the filter, it reduced.
If money no problem it is also good to have faster Lens, because you still can set to F4.0 with a F2.8 lens, but reversed is not true.