Well it was severe enough that I totally gave up on my unmodded 10D and got a modded 450D to handle H alpha. The difference for the 10D is with its original (bonded in) filter the transmission of H alpha is ~15%. The modded 450D has H alpha transmission >90%. Thats exposures at least 6x shorter to grab the same amount of H alpha

Zambonni's comparative images are a very good gauge. The Veil Neb is pretty well instructive. Without the mod you certainly get the OIII green light, but in comparison the H alpha light is pretty well non existant. With the mod in place the levels of green and red light are much of equal strength.

Then again I've been imaging for 5 years and avoided red stuff. There is plenty of other satisfying subjects (and they are usually things that others pass over in the quest for glowing red gas)