The reason that the 5GHz. range is as long as the 2.4GHz. range is just physics. As the signal frequency goes up, it's susceptability to being attenuated by walls, floors, and other objects, etc., and just plain attenuation over distance increases.
As far as increasing the power on 5GHz., keep in mind that you have to double the power to increase the signal by 3db., so I wouldn't plan on that helping a lot, although you could play with the transmit power (be careful about exceeding the maximum or overheating, though) and see what happens. Just be aware that 5GHz. signals have less range than 2.4GHz. signals inherently. That's also one of the reasons why the 5GHz. band has less WiFi interference, your neighbor's 5GHz. router signals aren't likely to make it to your house *smile*.
As far as increasing the power on 5GHz., keep in mind that you have to double the power to increase the signal by 3db., so I wouldn't plan on that helping a lot, although you could play with the transmit power (be careful about exceeding the maximum or overheating, though) and see what happens. Just be aware that 5GHz. signals have less range than 2.4GHz. signals inherently. That's also one of the reasons why the 5GHz. band has less WiFi interference, your neighbor's 5GHz. router signals aren't likely to make it to your house *smile*.