I'd go (actually, I have, twice) with 15" wheels instead of 16" wheels. The 15" wheels will be lighter, handle as well, and give you a slightly better ride than 16" wheels.
Miata's really do best with smaller wheels. These days it's next to impossible to find a good tire for 14" wheels on a miata, but there are still plenty of 15" tires. 15x7, 15x7.5 or even 15x8. I'd run a 205 50 15 on all of them, but you could run a 195 50 on the 7".
You may want to look into a foamectomy. You take the driver's seat out, remove the upholstery and do some carving on the foam in the seat back and bottom. It'll give you a lower seating position and a bit more lateral support. You don't need or want to take too much out of the back. Actually, there's a very easy piece that you can take right out of the center that's not even attached. You can take A LOT of foam out of the seat bottom and still be comfy. I'd suggest that you remove the foam from the bottom of the pad. That way the foam still holds the upholstery up when you get out. You can also put some less dense foam back in if you want.
I performed a foamectomy on my old '97 miata and a more mild one on my current '04 (different seats, harder to work on) and it makes a big difference in comfort and support (at least it did to me)
I'm on my second miata, and have had 3 in the family (wife has one). All three have or did have 15" wheels.
Here's mine
My previous miata (black) before I lowered it and took of the spoiler, and the wifey's car (green)
This is a good place to get wheels for your miata no matter which size you get. I've done business with this place a few times. They have a large selection of wheels, tires, and lugnets that are miata specific (size, bolt pattern and offset).
http://www.good-win-racing.com/