Don't get trapped into the marketing and the claims of "hi power". They at least are good enough to not lie and say that it really only puts out 28db where the legal maximum is 30db. Pretty much any adapter that puts out the same power will give you about the same coverage. The reason they don't sell the 9db antenna with the unit and sell it as add on is that the combination is illegal in most counties because you are now at 32db of power.
Still it likely will make little difference since the you generally must upgrade both ends and you will have ability to do anything about the router end.
The largest issue you will have is the walls of building both on your end and the far end as well as trees and other obstacles absorb a lot...