I measured my power consummation at the 230v side (so not to accurate) but I notice that when changing txpower from 0 up to 23 I could see and increase in power usage.
However going higher that 23 did not give and power increase so it kind of looks like that 23dbm is max on my card atm.
Will try to do same test again when I get my usb measuring device.
For testing I used "mdk3 mon0 a -a <mac>" (my own wifi mac)
txpower 0 gave 2.2w
txpower 20 gave 3.2w
txpower 23 gave 3.6w
txpower 26 gave 3.6w
txpower 27 gave 3.6w
txpower 30 gave 3.6w