Definitely true. The day I mounted an MR16 DIY light with an old 12V battery I noticed this, and as I've gotten better and better lights, it's only gotten more pronounced. I've been thinking about getting a MagicShine 880 (2K lumens, dual emitter) but the 900 that I have (450 or so lumens) is pretty good.

I think that beyond a certain point, you don't gain anything. Some people would not notice you if you were riding with enough lighting to catch bushes on fire.