What Fuel Octance do you use?
 

1991 Mazda RX-7 N/A

I've been filling the FC with highest Octane Rating available at any gas station I pump at

I was reading some RX-7 information on http://www.aaroncake.net/RX-7/newowner.htm about fuel and it says "Octane requirements for non-turbo (naturally aspirated or NA) cars are very low. 87 octane should be used if available. Always use the lowest octane fuel you can find. Higher octanes will result in a noticeable power loss, excessive carbon build up, and poorer mileage. "

I'm thinking I need to start pumping 87 Octane into the FC now

yes, use the low octane

Yep. This question has been asked a thousand times. The answer is still, not unless you need it.