I believe that DD-WRT (and HyperWRT) supports the same power boost. As far as the optimal power output, I don't know what works best for the card. I believe the default for OpenWRT is set to somewhere around 80mW, but you'd need to verify that.
Certainly there's little actual in-band power increase going from 50mW to 250mW output, so it looks like most of the 200mW worth of extra output is ending up outside the useful band rather than inside it.
You might do better with a higher gain antenna than with boosting the power output. You'd increase the ERP (effective radiated power), as well as boosting the sensitivity of the receiver. It really depends on the shape of the area you want to cover, as well as the materials used in the building.
Any views, thoughts and opinions are entirely my own. They don't necessarily represent those of my employer (BlackBerry).