The difference is is that the higher octance gas has a higher "flash point" than the lower octane gas. That is a critical and important difference. The octane in a gas is a measure of how hot and bothered (compressed) a gas can get before it ignites. When you run a high-compression NA engine (like the IS, although the IS does not have a really high compression engine) or a forced-induction engine (turbo or super-charged) you really want all the octane you can get. If the engine is knocking or is trying to prevent knock (via on-board systems in newer cars or aftermarket devices like MSD or J&S) it is retarding the timing. If you (or the engine) are retarding the timing, you are not getting the maximum power out of the engine.There is no difference between the 87 and the 92 octane gas, the only thing that the 92 will do is prevent knocking better than the 87
Modern cars don't all have the same compression ratios. The higher the performance figures for a given size displacement engine, the higher the compression ratio is likely to be when normally aspirated. However, many modern cars have their own knock sensors and can retard timing when knock occurs.with the compression ratio modern car uses, you usually don't have to worry about knocking under regular driving condition. The only engines you will have to worry about are the force induction engines.