AFAIK, as long they still sell the devices, the firmware must ensure its compliance, otherwise inventories might be seized.
I've seen the form the FCC asks them to fill. It's not reassuring.
I bet the point of view of Asus is simple: they will release new firmware for devices as long as feasible (I guess depending on how much of the code can be ported to or from newer devices). That is a blessing for the end user, many major router manufacturers stop firmware update support faster than lightning (e.g. Linksys).
To release new firmware Asus will at some points be forced to implement changes due to updated laws.
It is to the end user to enjoy the benfits such as security updates of the new firmware, including maybe the legal drawbacks.
No one will keep the end user from using older firmware if that better suits your needs.
Regarding TX power of a router: increasing the TX power only really helps if you also increase the TX power of the other end: the Client device. With many Client devices there is no way to increase the TX power. The number of bars for signal strenght (depending of the TX power of your router) is absolute no garantee for a better link or higher throughput.
Router/antenna locations, free line of sight and avoidance of obstacles are the major points of attention for a good link.
Radio wise we still talk about micro low TX power (80 mW or 120 mW is really a minor difference, 50% increase of power does
not stand for a 50% longer range), with relative poor antennas and RF circuits (due to the pricing and mass production). Increasing power may simply have technical drawbacks (such as temperature issues) and is absolutely no garantee for better signals.
Those who have ever built their own radio transmitter may wonder how it is ever possible that wifi works so flawless.
There are dedicated high power wireless devices on the market, with usually also better antenna techniques and another pricing level.