Lets break some of this down. My BS detector has been going full bore on a lot of this.
1.) Will running lower octane fuel in your car make it harder to start? NO! This is a wive's tale.
2.) Why is detonation bad? Because the increased PRESSURE due to combustion taking place before (TDC) TOP DEAD CENTER of the piston's travel up and down the cylinder can stress the connecting rods, sometimes bend a rod, cause crankshaft problems or (most commonly) pound the ring lands on the piston until the fracture and then your piston ring shatters and scratches the bejezzus out of the cylinder and the metal shards goes out the exhaust valve hopefully without to much damage.
3.) High altitudes effect on compression ratios and octane requirements. Higher altitudes DO lower the OVERALL effective compression ratio even in a TURBO car but NOT to the extent that normally aspirated cars will notice it. The only exception to this is IF the boost control system is REFERENCED to ABSOLUTE pressure rather than atmospheric. Most boost controllers are referenced to atmospheric rather than absolute. There is also a direct relationship also with temperature and altitude. Higher temps and altitude = less dense air = less OVERALL effective compression ratio = lower performance (usually). SO... you could run lower octane fuel at higher altitudes with less detrimental effect. Still wouldn't suggest less than 89 octane in a turbo subie. Mine detonates on 93.
4.) Fuel mileage will not go up 2-4 mpg if you run 93 vs. 87 octane. This is PURE BS! The extra octane ONLY comes into play at high boost and load conditions. At normal cruise, i.e. 70-80 mph the subie motor could run on 85 octane without much issue. Another way to suppress detonation is with water or alcohol injection.
5.) Lower octane fuel actually has a HIGHER energy content than higher octane fuel.
6.) Based on #4 above, if you drive mostly highway and rarely accelerate hard, you can easily and SAFELY run lower octane fuel. If you like performance like me, you will always be hunting done the race gas to improve the performance IF and WHEN you have the mods to truly take advantage of it.
7.) Higher octane allows you to run more ignition advance which gives you better throttle response and efficiency but only up to a point. It also allows you to run more boost assuming you have the mods to take advantage of it. Hi boost with a lot of ignition advance is a sure recipe to invite knock (pinging, detonation, etc.). This is the ENEMY of turbo and supercharged motors. Left unchecked it can destroy your motor in a half second or less.
8.) I could go on and on about this but hopefull you get the idea.
Tim