I just moved to Colorado. Driving at between 5000 and 11,500 feet daily. Wondering what I can do to my engine to make it run optimally at this elevation. I was considering a drop in K&N. I have also been using at least 91 octane which seems to help...

Typically higher octane fuel gives most benefit at lower elevation.

In fact, the higher the elevation, the lower the octane requirement as the O2 levels thin out.

Anyways, breathing at high elevation is going to follow the same rules as breathing at lower elevations.....but your gains will be smaller due to lower O2 levels. Volant is a good idea, NISMO catback is a good idea, Headers and HFCs are a good idea.

But the best improvement you can do for breathing at elevation is to add forced induction, i.e. a supercharger.