A Sveasoft firmware upgrade will turbocharge your router delivering over one hundred feature upgrades, improved stability, and increased wireless power and range, including a power increase up to 1000% over stock firmware.
The stock firmware radio output power is fixed at 28 mw by most router OEMs like Cisco/Linksys. The Sveasoft firmware allows adjustment of the power from 0 to 1000 mw.
The legality of changing the power depends on the local regulations and the antennas used. In some areas using the full 1000 mw and the stock antennas is illegal. In others it is still far under the allowable limits.
EU countries limit the maximum radiated power to 20 dBi. The stock antennas are 2.2 dBi and this means a maximum power setting of of 17.8 dBm for the radio. The maximum radio power setting is thus 61 mw for a total of 20 dBi.
North America limits are much higher and the maximum allowable radiated power for point to multipoint links is 36 dBi. In the US with the stock antennas and 251 mw you will produce just over 26 dBi. Thus you can add much stronger 12 dBi antennas and still meet the 36 dBi limit.
Higher power settings do not generally mean higher throughput. But for long links they can mean the difference between a working link and none at all.