I average about 28 most of the time, even with some agressive driving, but I do get some highway driving in there as well.
I don't think anyone will bitch about the CAI or reflash, unless it causes a problem with your car and they therefore have to void your warranty. The chances of that happening, though, are pretty slim. I got mine through Mile High, and I think that they might be somewhat receptive to mods too but haven't really checked into it since I'm most likely going to leave my car stock. The car was running good today though and I popped the hood and the intake manifold was very hot, so I'm sure a CAI would help quite a bit, but I don't think I want to make the jump. The reflash would also be good but telling my wife that I can't drive for 3 days would probably piss her off... I did all my mods on my previous civic so I kindof got that bug out of me.