Not much you can do. The signal levels are regulated by the government and even if they weren't you don't have any control over the central router.
Make sure on your extender it is running only as a client you do not want it run as a repeater...ie you do not want it to transmit a duplicate radio output. In effect you have a wifi nic you connect via ethernet. It likely works slightly better than a internal card because it gives you more placement options. It also likely transmits at full legal power which many USB cards do not.
You might try a directional bridge. This is very similar to what you are doing except it concentrates the signal into a narrower beam. This works well when you have direct line of sight and when...