You will introduce some understeer, mostly in low speed stuff. But the benefit is in being able to get on the power a lot earlier when exiting a turn.

If the understeer is excessive, you can tune it out with alignment. (more front camber if you can get it)

For strictly autocross purposes, I believe the philosophy is to put on the stiffest front bar you can get. The Miata doesn't have a lot of power, so you want to be able to make the most of what it has.

Best of all... a swaybar is easy to change and if you don't like it, you can get most of your money back by selling it.