Valkyrie

Even some exterior mods are justifiable for race and track cars...splitters, big wings, rear diffusers, big wheels (look at GT cars), widebody kits, front air dams, etc.

Wheels are good because of the weight and extra rubber you can put on the road, plus looks, and the faster response you get from lower profile tires. Using spacers to widen the tread also improves handling, although at the cost of wheel bearings (theoretically).

An exhaust is good because it's lighter, gives power, and sounds better. Plus, a good aftermarket design looks better and has better construction than stock mufflers.

Also, an LSD is only important for heavy track work and drifting, but it does help on autocross with slaloms especially. At least a viscous is better than an open diff.

Suspension mods lower and stiffen the car, which reduces body roll and increases stability at the cost of ground clearance and comfort.

Of course, you only need to stiffen the suspension when you either lower the car (to prevent bottoming out) and because you have more grip (which means more body roll).

BUT, rebuilding, restoring, refreshing is the first thing that needs to be done before you mod a car.

Get a full bushing set (Urethane, or hardened Mazdaspeed rubber), new engine, transmission, diff mounts (stiffened Mazdaspeed parts are good), new brakes, clutch, paint, transmission ,engine...

You can stiffen the body up with tower bars, have your interior cleaned or redyed, get a dash carpet to cover up any cracks (and eliminate glare which I HATED on my FC...).

With a car that's been on the road for 20+ years, the list of things that could have gone wrong before they went right is pretty long.