OK so for what it's worth, I think I have this figured out finally. If you want to connect in 2.4Ghz mode @ 300Mbps, you can by selecting 40Mhz for channel width, but this is only an option for "N only". I still can't seem to get it to connect @ 300Mbps when selecting 20/40 atuo. When I do, it's always at 20 despite having only my one N computer connected.
So, maybe I'm beating an almost dead and worthless horse, but can anyone figure how the router decides to use 40 in the 20/40 auto mode? Don't say put it in 40 only mode cause I have many G clients that sometimes connect!