Here in Phoenix, the highest available is 91. They quit selling 93 years ago.

You should be able to tell the difference between 93 and 87. Most engines these days have knock sensors that tell the computer to retard the timing a bit when knocking occurs, and what you notice is a bit less power. The knock sensor will pick up the knocking before you hear it, so you won't hear it if you put in 87.

I suspect that you should feel a power difference during part throttle acceleration, since that is when knock is most likely (because that is when the engine experiences the highest load). Full throttle should feel different as well, but unless your engine is fully broken in, you may not be able to tell there.

There really is no harm in using lower octanes fuels, you just get a reduced performance out of your engine.

Why you feel no difference is beyond me. You really should!