After reading about Torsen LSD's and how they act like an open diff if one wheel is in the air, I was searching the internet in hopes of finding a better option for the front.

Turns out that a Japanese company called OG Giken makes a product called Super Lock LSD. It's supposed to be "the best LSD available" with less heat creation and 100% lockup capability. They miniaturized the componentry to allow them to fit 28 friction plates into the unit.

They make one that fits 94-05 1.8 Miatas, so I'm thinking this may be a drop-in for our front diff? The downside? Costs something like $1300 to $1500. Ouch!

http://www.osgiken.net/products/superlock.html

http://miataroadster.com/miataosgiken.html