11n-only might help a bit if all your devices, and guests' are 11n, which they probably are. 11g clients would be 4 years or so old.
You may have negative impact if you have a neighbor with 11b/g and IF that neighbor is near your channel, and IF the neighbor does extensive streaming via WiFi.
Changing channels to 1, 6 or 11 - all depends on if you have a neighbor that streams video a LOT on or near one of those channels.
Try speedtest.net. Try several different servers to find the fastest. Do it on both WiFi and a PC with wired connection to router. See if you can get near the speed promised by your ISP for down and also up.
WiFi speed will vary a lot by which device is used and of course, the signal strength at the client, and also at the router (though the latter is rarely displayed by routers' admin tools).