Open telnet (you can turn it on in the administration of the router)
before you do anything you might want to check this value and make note of it:
To change to us:nvram get pci/1/1/ccode
nvram set pci/1/1/ccode=US
nvram set pci/2/1/ccode=US
nvram set wl0_country_code=US
nvram set wl1_country_code=US
nvram set regulation_domain=US
nvram set regulation_domain_5G=US
nvram commit
do a reboot and your router will allow 500mW for 2.4GHz and 250mW for 5.2GHz connections.
Please keep in mind that the US allows different network channels, especially for 5GHz: http://en.wikipedia.org/wiki/List_of...2Fh.2Fj.2Fn.29
to go back to eu mode is easy:
nvram set pci/1/1/ccode=EU
nvram set pci/2/1/ccode=EU
nvram set wl0_country_code=EU
nvram set wl1_country_code=EU
nvram set regulation_domain=EU
nvram set regulation_domain_5G=EU
nvram commit