can't get connected at more than 54Mbps on 2.4 or 5Ghz?

I've got tomato installed on my Linksys E3000 and although I am able to get connected via my laptop or phone on the 5Ghz wireless, I can't get over 54Mbps (with a real throughput of around 15Mbps) on either 5Ghz or 2.4Ghz, which is a little frustrating. I've tried the 40Mhz channels and upper and lower sideband, but nothing seems to work. is there some other setting I'm missing?

is there a better firmware for me to be running on it? the current firmware on it is: v1.28.9054 MIPSR2-beta...

can't get connected at more than 54Mbps on 2.4 or 5Ghz?