Think about the amount of mass an ATV shock has to control vs. the mass a car shock has to control. The forces required of an ATV shock are relatively low. I have no reason to think one would be anywhere near capable of controlling a car well. I could be wrong... (there's a first time for everything.)
Now, what are you doing with Miata parts? You have to be careful of your use of the word "torsion bar" here. Yes, a sway bar is technially a torsion bar, but since your car has torsion bars instead of springs, I can't tell what you're talking about. You using Miata sway bars? Explain...
I only mention the rear bar because all front-drive cars understeer and the cheapest way to fix that is huge amounts of rear roll stiffness. If you reach the point of too much rear bar (ie. its causing more oversteer than your drivers are comfortable with), then you can think about front bar. Until then, focus on the back.
Make the bar very adjustable, too. This will let you find the balance you want much more quickly. Back when we had a wide talent spread among our drivers, we'd run different bar settings for different drivers.
-Dave