i recommend to you to use a higher ISO and then get Ninja or neat image to clean up the noisy in the photo. Better then to get a lens with a big whole cos it will cost a even bigger whole in your wallet.
Since most people is worried about the noise level in higher ISO, so, which do you think is of better quality:
1. f4.0 lens (slower) with ISO 1600 (17-85 IS)
2. f2.8 lens (faster) with ISO 800 (14-54)
Having a faster lense means that you have luxury of going in different light situation without sarcificing the quality of the picture. If one is talking about of depth of field, a F2.8 will giving u a better out of focus background(shallower) than a F4. There will be times u need faster shutter speed and boosting ISO will give u grainy pictures, to capture motion for eg., that where a faster len come in.
Of course, having a faster lense also means that you pay more and carry more weight.
I agree that DOF is deeper on the Olympus system, but deeper DOF is sometime useful. For example shooting macro shots, I no longer need to struggle up to f22 to get sufficient DOF, usually f11-f16 will do. This give me plenty more light to work with.
But, I don't agree on how you phrase it - I think, it is misleading as people may think that the Olympus system is darker when u say f2.8 = f5.6. Rather, is actually because of the shorter focal length to achieve the same field of view (fov).
I would rather say, for the same fov, 300mm film = 150mm 4/3. That's the reason why, the latter has deeper DOF.
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.