I think the channels are irrelevant, as I'm working in the 5Ghz range, which a) doesn't have channels 1-11, and b) has non-overlapping channels. 
I have progress! Turns out I WAS having a double-NAT problem, so I was able to call Comcast and switch my modem to bridge mode (kinda wish I could have done it on my side, but the guy was very nice and we were able to get it done with minimal hastle). So, only one NAT and one DHCP (as far as I know)! My speed HAS gone up significantly, now up to 30Mbs over Wi-Fi (via speedtest.net, the Xfinity speed test is still giving me 30Mbs as it was beore). Technically, I should be able to squeeze out some more juice from this thing, but I think we might be starting to run out of ideas. I'd be willing to call this one solved, but if anyone has any idea as to why I'm getting 10-15Mbs less than the theoretical speed from my router (taking into account standard overhead) under ideal circumstances (right next to the router, no nearby devices, no other 5Ghz Wi-Fi in the area, etc), I'd be happy to try anything.
Regardless, everyone has been a huge help, so thank you!!!