Following instructions are provided without any warranty, and may even get you in trouble legally. The instructions are provided for testing, learning, preventing e-waste, and should be use with care. We (including contributers + commentators) are not responsible for any damage to your device(s) or any legal issues.
Instructions have been moved to https://github.com/francoism90/asus-router. :)
These settings work great on my RT-AC66U (Asuswrt-Merlin 374 LTS Beta - 374.43_53D7j9527)
I get about -20dBm on 2.4GHz and -10dBm on 5Ghz compared to stock.