Hyper Ground Systems, the "generation" tuning system ???? WTF??? I guess if you are the right generation than it will make hp for you. Pretty much bunkum'.
The Miata has the battery in the trunk, and and a very tiny battery to ground cable, so it is about as ground challenged as a car can get. I have never seen a doucumented case of a Miata that was running properly gain a single whp on a dyno from a ground kit. Sure if the ignition was misfiring due to a ground you could see a gain if the ignition stopped misfiring otherwise it is all bs.
I actually added a big flat mesh type ground cable form Wal-mart to my Miata and saw a benefit. I had some intermittent issue with my tach dropping rpms. The car ran fine, no misfiring but for some reason after the blower was installed the tach was erractic. It seemed to be worse in high humidity, or rain, suggesting a grounding problem. I went to the battery cable section at Wal-Mart bought a less than two dollar cable, attached it between my intake manifold casting, and the chassis, and my tach issue went away for good. I attribute the ground problem to all the splices required for my piggy back computer to control fuel under boost.
I seriously doubt the 3 has any designed in ground issues... all the electronics seem pretty robust to me. You can spend your money but if you feel a gain it is probably all in your head. If you really think it is worthwhile, get your car on a dyno, do a baseline, then install the ground wire kit and run another pass or two. We would all love to find a couple of cheap whp. Do the community a favor be a pioneer.