My vote goes to the Sypder grill
(Jokes aside - it does wonders for the front-end)
Judging by your comments on the "new and improved smoothness™", it would seem the natural way forward for this car is the Ohlins / supercharger / best Goodwin exhaust combo - promote the feel of a high-end GT car by modifying the few weak points where Mazda catered to the masses - then perhaps spend time refining that package more completely by testing various options (braces & even bushings), so we end up with the best compromise of daily / weekend toy.
Basically, a test mule for "wife approved" mods on the soft top for those who prefer it over the RF...
(I'm assuming you still have an ND1 you can use as a proper track toy?)