Well, for a little calculation... The power usage of the pure transmission power of the Wi-Fi... Considering the change from 100 mW to 10 mW is 90 mW, and the nominal battery voltage is 3.7 V (varies from 4.2 at fully charged to around 3 V when empty). So, 90 mW will on average take 90 mW / 3.7 V = 24 mA. Given the battery real full capacity is about 1260 mAh when new, this gives a time of a bit over 2 days. Of course everything else piles up, so if a random browsing etc on 100 mW Wi-Fi uses the full battery down in 8 hours of active time, it uses about 160 mA. So, in ideal case that the Wi-Fi were transmitting at 100 mW continuously, which probably isn't the case ever, you might increase the time to 9 hours. Or perhaps if you transfer data at full speed on Wi-Fi it will use that. But then the whole thing will probably use much more battery in any case lowered the real benefit of lower transmission power in ideal conditions to more like half an hour.

The extra consumption of using 100 mW transmission power on Wi-Fi is quite small in the worst case scenario of continuous full power usage. And while it may cause lowering of signal quality when further away from the router etc. Those might account to lower transfer rates and more usage elsewhere on longer bursts etc. All in all, you might as well be losing battery time with 10 mW change in addition to possible problems with Wi-Fi connection alltogether and likely lower transfer speeds unless you are close to the Wi-Fi router.

As of now with the Wi-Fi bug I am experiencing (using about 180 mA no matter if Wi-Fi is on or off after having used it even once - only to go back to normal with reboot) there is no way for me to test the real outcome of the settings. And at least for that bug, there is no effect for that setting.