I have done the deckplate mod and I'm not worried about it. The $200 Air intake kit would be just as exposed as the deckplate mod, if not more so.
I also run the K&N air filter and I like it. I noticed a little increase in power and it has driven the same now for a few months. My guess is that if you think that you get an increase in the beginning but then you loose it, you just are getting used to it and don't notice it anymore.
Who knows, I probably don't know what I'm talking about, but I'm happy with my mods and that's what counts.