Just because they can set their fantasy levels doesn't mean that the hardware is actually transmitting that powerful.
There are also several changes in txpower handling between 10.03.1 and current trunk, then there are varying EEPROMs in the wild, different configurations (user regd on and off) and finally multiple hardware revisions.

So what you mean is that we shouldn't bother about getting more than 17dB since the hardware do not support it?

Exactly.
11g OFDM on 2.4 GHz ~ avg. 17 dBm peak power per chain (report 15 C, pg. 66+)
11a OFDM on 5.7 GHz ~ avg. 19 dBm peak power per chain (report 15 C, pg. 168+)

One other small item to consider, this is two way communication, the other end (ie. laptop or computer) would also require improvment in signal strength.
Better antenna and relocation of the router and laptop is an optimum solution.

This is (cough! cough!) a ms link but it illustrates the point. http://www.microsoft.com/athome/setup/wirelesstips.aspx

~john

I am still stuck
I have an WNDR-4300 and it is stuck at the following:

wlan1     IEEE 802.11an  Mode:Master  Tx-Power=14 dBm   
          RTS thr:off   Fragment thr:off
          Power Management:off
          
wlan0     IEEE 802.11bgn  Mode:Master  Tx-Power=19 dBm   
          RTS thr:off   Fragment thr:off
          Power Management:off

and I cannot get it any higher.

The stock firmware offers a lot of more range and stability for the wifi link (at great distances).

config wifi-device  radio0
    option type     mac80211
    option channel  6
    option hwmode    11ng
    option path    'platform/ar934x_wmac'
    option htmode    HT20
    option country    DE
    option txpower  20
    option distance 35
    list ht_capab    LDPC
    list ht_capab    SHORT-GI-20
    list ht_capab    SHORT-GI-40
    list ht_capab    TX-STBC
    list ht_capab    RX-STBC1
    list ht_capab    DSSS_CCK-40

config wifi-iface
    option device   radio0
    option network  lan
    option mode     ap
    option ssid     xxxxx
    option encryption psk2
    option key    xxxx

config wifi-device  radio1
    option type     mac80211
    option channel  44
    option hwmode    11na
    option path    'pci0000:00/0000:00:00.0'
    option htmode    HT40+
    option country    DE
    option txpower  18
    option distance 35
    list ht_capab    LDPC
    list ht_capab    SHORT-GI-20
    list ht_capab    SHORT-GI-40
    list ht_capab    TX-STBC
    list ht_capab    RX-STBC1
    list ht_capab    DSSS_CCK-40

config wifi-iface
    option device   radio1
    option network  lan
    option mode     ap
    option ssid     xxxx_5g
    option encryption psk2
    option key    xxxxxxxxx

try this and read the readme before !

http://luci.subsignal.org/~jow/reghack/

ok, did the patch and it seems to work for the 2.4GHz wifi when I set the reg domain to US but the 5GHz radio is still limited to 14dBm even with the patch applied and iw list listing all channels unlimited...
and since the 5GHz range is most concerning this is just a very limited fix so far... but thx a lot for the quick answer