And to repeat: It's not a common occurrence for me, but if I'm near the router and get only 100mbps, it's probably because I'm on a 2ghz channel.
The only time it was a real issue was with my desktop, which is not hard wired yet. I bought a new AC card for it and it kept defaulting to 2ghz. I disabled 2ghz from the computer, and now it works fine.