I just tried doing this. There is a diagram with 8 different designs of these things! Crazy!
I want the ones exactly like the OP has, that screw on to the original studs and have studs built in. I have some basic 1/4 inch spacers and not being hub centric they vibrate and I get more road noise and would prefer these hub centric kind
When going through the links on the Eibach website I found this model# on Summit racing website: EIB-906150271 for 15mm spacers for the mazdaspeed3, but they are $139.96 for only 2 of them! You'd be spending like $300 shipped for all 4, which is ridiculous