As Michael Tyson had said, the OSX driver doesn't use the full power of the RTL8187l. The reason why is a region-locking issue. In the US the TX power is limited to 27dB. In other regions, such as Bolivia, you can use the full 30dB power.
When I run Backtrack Linux (in Virtualbox) I've been able to change the region and TX power setting:
iw reg set BO
iwconfig wlan0 txpower30
I originally found this on this blog:
In Mac OSX there is no iwconfig command and ifconfig doesn't allow for the commands to change the region and power setting as far as I know. Anybody have any ideas?