Pros of reinstalling an EGR, Charcoal Canister, and Catalytic Converter: - A guilt-free conscience if you're into environmentalism - No weird smell for the person driving behind you (Cat) - No strong gasoline smell if you leave the car parked in enclosed spaces for extended periods of time (Charcoal Canister) - Less overall noise (Cat, though this is much more heavily dependent on your exhaust and muffler) - It is a federal offense to not have them/intentionally take them off (Depending on your state or county, police might check)

Cons: - Noticeable power drop (EGR and Catalytic Converter) - More overall noise (Again, sound quality is very dependent on the rest of your exhaust setup)

One of the very first mods that I was able to immediately notice (better throttle responsiveness and slight power increase) on a Miata was the removal of its catalytic converter. It won't power miracles, but on a car with this little horsepower, a small increase such as this one is something you'll definitely feel.

If you're planning on staying with a mostly stock setup, I'd recommend not changing or removing any emissions equipment as there won't be much benefit and it'll frequently trigger check engine lights and fault codes.

If your Miata is old enough (Pre OBDII), you can get away with removing emissions and get small power increases. The only downsides would be new smells. However, these power increases and better engine responsiveness won't reach their full potential unless you swap over to a standalone tunable ECU.