I posed the same question on another forum, and got a response from another member who went from a 99 Cobra (close to same weight/hp ratio) to a spec Miata; here's his reply:
"I'd be glad to offer any information I can. There are a few big reasons to buy a miata, and this is no particular order.
1. Burn rate is WAY lower...
My Miata gets 16mpg on track whereas the Cobra was somewhere in the 3.5 range. In three seasons I'm on my fourth set of front pads, and my third set of rear pads. One pair of front rotors, a 60 dollar napa radiator and other various trinkets....for around 15,000 track miles. Not 1,500....15,000. I did replace my 1.6 with a newer 1.8 engine, but the 1.6 ran fine...I just wanted a little more scoot out of the corners. There is nothing out there that can even come close to this level of reliability and affordability for the amount of seat time i'm getting. I have three set of tires in rotation. Hoosier Wets, Practice rubber (usually tires too old to TT on) and my TT rubber (usually low cycle count Hoosier SM6, R6 or A6, but V710 is also very good.) The wets are on their third season, the practice rubber is from last year, and the A6's are new this year.
This kind of savings allows you to spend less time fixing the car/replacing parts, and more opportunities to be out there driving.....which brings me to my next reason.
2. Driving a Miata is more about you than it is about the car.
With the exception of of having 300+ hp, this car does everything better than every other car out there. It is so well balanced that you will naturally be taught by the car what is the best way to drive it via osmosis. You will find that you will begin to use the brakes less and less, until you are using them very little, or not at all in some places. once you discover that level of momentum driving, you will be making passes on much faster hardware, mostly because folks with 300+hp cars never really need to learn how to turn...they can just stomp the throttle and power out of the turn they just blew. In your case, if you blow a turn....you blow the lap. The focus becomes being technically proficient in every corner....there are no throwaways..
Once you learn this method of driving, then driving any car becomes easier, because you can sniff out the proper lines for the machine to be on.
Case in point:
I was at Mont Tremblant in Quebec a year or so ago at an event. My buddy who has a C6 corvette was at the same event. We were both datalogging our laps. I was running 2:03 lap times, and he was running 1:58's.. So he was bragging that he was faster.
I asked him to pull up the datalogs to show him something. I selected Turn 1...where at the end of the front straight I reach about 100mph...and he reaches about 120 or so. Then I told him to move the cursor through turn one....where he brakes and turns in at around 60mph.....and where I don't use the brake at all, and take the turn at WOT at over 95-100 mph. The corvette storms back in the straights and makes up for the time.
in short...it takes serious ballsack to go fast in a Miata....because to not have it will mean you will be slow.
It forces you to drive the shit out of it.....just to put up normal lap times....but you become a seriously proficient driver in doing so.
3. Definitely buy something that is already sorted out.
There is no reason to buy something that isn't already built up and ready to drive. Less cash spent buying parts is more cash toward seat time.
I would recommend a car in the 94-97 range with a 1.8......or newer. The 1.6 cars are fun, but you will out grow it just a little faster."