Think of it this way...the only thing the octane number does is tell you how fast the gas will burn. That's pretty much it.

The higher the octane, the slower it burns...

The slower it burns, the sooner the spark has to fire to get it going in time to push the piston with the explosion. (Timing)

If an engine is designed to adjust the timing to account for this, then higher octane fuel may improve performance.

In the X - the regular grade gas is pretty much the hot ticket.

If you drive all over the country...what is considered to be "regular" will change, mostly based upon the altitude of the region.

Regular gas at sea level areas (NJ) is around 87 octane...Regular gas in Colorado is more like 85 octane.

Thinner air has fewer molecules in the same space...so, to compensate, the gas needs to burn a bit faster for the same timing settings to work.

In other words, if you are going up into the mountains, and the engine starts to feel weaker, putting in a higher octane gas will make it worse, not better.