Hey guys! Jumping in a bit late...
It's actually 3.25" but you all know me to be a stickler for details

Yes, all lowering springs for the Miata are 3.25", to fit the stock perch. Tein S-tech, Racing Beat, Eibach, Tokico lowering kit, stock, FM - all will work.
The 2.5" kit will take any race spring, including some that are also excellent quality but much less expensive than Eibach, Swift, or Hypercoil. The QA1 available from Summit Racing and Suspension Specialist Springs avaialble from PitsStopUSA are my top choices. I just got a batch of QA1 400 7" and 300 7" springs for a few customers getting complete coilover kits.
I will be offering springs to make full kits but don't want to carry everything under the sun. Easy enough to order and have it delivered in a week, that way it's exactly what you want. If you buy a complete coilover kit, including springs, perches and FCM mounts, you'll get bump stops thrown in for free!
I'm glad others appreciate the variety of options! I'm going to be cleaning up the website to simplify the interface but I'm happy with how it's progressing.
Shaikh