Hi Exfso,
I'm definitely no expert at imaging, but I thought the logic went something like this. A UHC filter reduces the amount of extraneous light, therefore, in a proportional sense, you're getting more of your nebula light - that'd be good for photography. There is also an important distinction to be made between emission and reflection nebulae though; one goes well with a UHC the other not so well. Don't quote me but I think UHC's perform better on reflection nebulae. Of course in absolute terms ANY impedence will reduce the rate at which your 40D gathers light and therefore increases your exposure time.
The little I know came from this site:
http://www.company7.com/baader/options/uhc-s.html
Hope I've finally helped someone else.
Mike.