HIGH Altitude riding – Rockies

So I recently moved from Minnesota to Colorado and gotta say the riding out here is unbelievable. Steep, Rocky and technical so makes for fun and challenging riding. I have several questions for riding at high altitude.

1- really down on power at 10-13K feet compared to sea level. I have a 2010 and 2014 850 XP’s. I have 26 and 27 inch tires on the machines and also have an EPI clutch on the 2010 as well as an aftermarket exhaust ( swamp series exhaust ) and the 2014 is very stock except for QSC clutch and 26 inch tires ( Kenda Bearclaw HTR II) .

2 – right gas to use ? 91, 87 or 85 octane ??

3- gas additives to deal with all the ethanol is this colorado fuel ??

4 Octane booster of ANY value ?

5 – Air Filters ? Any value at altitude ?

6 – 2014 850 XP – I installed a QSC clutch from Ryan in MN, great guy… I got a little stiffer spring and lightened up the keys to 57.5 Grams and it seems to be more responsive.

7 – Sparkplugs – I am running the laser NGK in both machines.

Any help from all you high altitude riders is greatly appreciated.