I would think you shouldn't have problems with any channel since you shouldn't have too many base stations in your immediate area.
http://support.apple.com/kb/HT4459
I don't even see anything related to wireless except this security fix:
http://support.apple.com/kb/HT4298 and I don't see anything speed or channel related there.
The only thing I can think is that coincidentally someone installed a new 802.11n router near you or something else odd happened. Do you have the channel set to a fixed number or Automatic? With automatic, it should actually shift if it detects cross-traffic.