I have a OS Giken LSD in a 3.9 Differential, with correct speedo gear, that was professionally setup by Hartzel Automotive of Monterey (Builder of Spec Miatas). These LSDs are simply the best you can get for the miata and was an excellent addition to my 271whp SSM Miata. This diff only has a few thousand miles and probably 10 AutoXes on it, practically brand new. To replicate this setup, you'd have the $1,600 OS Giken, $400 for 3.9 and $300-$400 to have a professional shop set it up. Easily a $2300-$2,400 setup.

If you have a 4.10 Torsen you would be interested in swapping, I'd drop $400 from the price.

Price $1,600.

Located in Northern California, could potentially arrange swap in San Francisco Bay Area.

 

I have a 4:10 MSM Torsen with Axles if you're interested. I am local to you as well.