I haven't tried G only. I'll give that a go. I have some devices that support N (Xbox 360, iPad, laptop), so I thought I would use it since I finally have a router that supports it. The 5 GHz on the iPad has been mixed. It works well when I first connect, but then slows down/stops working. The 2.4 is more consistent. I haven't tested this with SB disabled, but then again, I really don't want to leave it disabled.
I read your posts and set a manual speed of 100/10. My cable is 30/5. Speed tests usually don't quite get to 30 down, but they are close. The up frequently is between 5 and 6.
I own my cable modem, it is the Motorola SB6120 SURFboard DOCSIS 3.0 eXtreme
Distance to my Tivo is about 30 feet or so. Through some drywall, but pretty much a straight line. I could try the router at a different height, but it is in the same spot the 4300 occupied. Streaming Netflix on that does not yield full bars and HD quality, like it used to. 6/10 bars with some noticeable delays compared to the old router. Didn't even think too much about it before. The Tivo reports much better signal quality than it used to - 96% vs 80%, but the throughput is obviously not as good.
The PS3 is at an angle and down from the router. Not a ton between it, but also not completely unobstructed. Again, it used to stream in HD, no problem. It would not work today until I turned off SB and then was still lower quality.
I do have a 360 in another bedroom on the same floor as the router. It seems to do pretty well streaming over the wireless N, but the kids have noticed a little bit more caching than it used to.