Answered my own question.
Since I've started the same discussion in the German section of this board, please refer to that thread for further inquiries.
Thread closed.
Thanks.
swiss_man
EDIT: Here my findings in short. (Please put any comments on them in the other thread. Even in English. Thanks.)
The command "wl txpwr1" returns garbage on WL-500gP V1.
But you can set with it the radio transmission power:
wl txpwr1 -o -m 23
23 mW seems to deliver best results. (Higher values may lead to degraded signal reception.)
This setting will last even after rebooting the device.
To ensure that it will not be changed by the web interface, you might want to set it there as well, even though I have not tested if it has any influence on tx power. For this, use following commands:
nvram set wl_radio_power_x=23
nvram commit
This way you will always see the current settings in you web interface. This is important insofar as after rebooting your device, the command "wl txpwr1" will again provide useless data, even though the applied settings for radio output control remain effective.