How to significantly lower HR[avg] while improving performance output
First off, understand that HR drops as you age, even if you stay in good physical shape.  Older riders will tend to have a lower HR.

In addition, HR drops as you become better conditioned to high-output activity.  Basically, as you suggest, the capacity of the heart and circulatory system increases so that sufficient oxygenated blood can be delivered to the muscles at a lower HR.

Also, as one trains, the muscles develop more anaerobic capacity, so they need less oxygen, and they develop more glycogen storage capacity, so they need less blood sugar.

Others probably have more specific suggestions based on research, but basically to train the circulatory system (and muscles) you keep repeating the conditions where you want it to perform.  Some accommodation occurs within a few days (mainly in the form of hormone levels and blood volume), while full accommodation probably takes 6 months or so of every-other-day full-out rides.