Exactly the same situation, hardware A2, firmware 1.12EU 2008/10/23.
I'm considering upgrading the firmware but not sure if it's really worth it. My DIR-855 runs absolutely perfectly with 2 wired desktops, 2 laptops using 2.4GHz, 2 laptops using 5GHz, a Wii and a PS3, and a number of phones, often with many of these devices simultaneously on and downloads at my full bandwidth of around 23Mb/s. No router restarts, no dropped packets, the router is really really stable, so I'm wary of installing a new firmware on it!
Do the latest firmwares have any stability or other issues? If things are good now is it risky updating? Also, can we export and restore settings between 1.12 and the newer firmwares?