The first most obvious answer would be to use a different RB....
Next is, if you are not using the other two WLAN cards take them out. Just because they are disabled does not mean that they are turned off... They are powered 'on' just disabled for use.
You could use lower power card(s), the R52 for example. Set the power out lower - just enough to meet your needs.
Connection tracking has really nothing to do w/power consumption... It does make the cpu work but the difference in power consumption is very small. Same for firewall rules and bridging - no help there.
I don't remember the complete specs -MT help out on this one, but I believe that the total power for the RB333 (cards and all) is something like 35 watts. The board itself I believe consumes a max of 15 watts before adding anything.
thom.lawless [at] rapidwifi.com