Props to you OP for having the composure to drive steadily and economically. I can't do that in my Miata
Regarding differentials and gearing, I installed a 3.9 differential in mine, while it originally came with a 4.1 (94 model, with the 1.8). The new diff is off a 10th anniversary NB model. The rev difference is not a lot but it makes for a tad quieter ride. In town however I used to drive almost everywhere in 5th gear, now I only use 5th above 40mph as if feels that I'm lugging the engine.
If you could live without having the top down, you could invest in a hardtop plus a hardtop spoiler. The hardtop spoilers for the Miata seem to continue the slope of the hardtop nicely for aerodynamics.
Later edit:
About the soot on the tailpipe, as far as I know Miatas are tuned rich from the factory. If you want to get that out of the way, take a look at Speeduino, open source ECU. Cheap, already available with plugs for the Miata wiring harness and tunable with the same software as Megasquirt.
You can also increase timing by rotating the cam angle sensor and see how that helps your mileage.