I would really love to see why you make these stupid comparisons. No **** a race-prep car will slaughter any street car, not even through a power comparison, but through weight loss, suspension upgrades, and a trained driver. Hey, since we're talking about this, let's compare the GTR to a stock Civic. I guess the Civic might have a chance, right?
Any serious race team that preps a car for a national race spec like GT300 or GT500 or Le Mans will spend a quarter of a million to over a million, mostly in just development costs. It doesn't even factor in the amount of time dialing everything in after it's fabbed up, or whether it's tuned to the driver's liking. Let's also not consider the cost of paying race engineers and professionals who spend hours and hours drawing everything up, doing all the math, finding out what oil weight is best to run, whether it will be the option of high displacement NA motor or a lower displacement FI motor. It wouldn't be a far shot to say that race spec Mazda probably has over 400 thousand dollars poured into it. It's a private race team, though, so I highly doubt they spent over 300, but I don't even follow this race league.
EDIT:
Another thing to consider is that this is BTCS, which is a touring car endurance series and divides classes into T1, T2, T3, and T4. Why would Nissan even put the Z into a Belgian race league? Also, to say it's a Mazda 3 is really just completely false, the body is the only thing retained, considering it's running a 20b motor. The GTRs which are run in SuperGT used VH46s or something, and now they have an even smaller motor with a lower displacement than the VQ37 that Z's have now. There's nothing about them that really says GTR except for the exterior.
BTW here's a zed with a 20b.
Here:
If you want a Z in general that will walk circles around this Mazda3 in terms of general performance, look at Xanavi Team's 350Z back when SuperGT was called JGTC. Comparing the cars is, again, an apples to oranges comparison. If there was a race where any team fielded 370Z in the same class as that Mazda, then you'd know what to look for. Nissan even offers the 370Z in a full RC spec car, capable of entering into the vast majority of GT classes around the globe. Slap on some bolt-ons, convert to E85, forge your pistons, rods, and crank, and you now have a high compression V6 that will make plenty of power. Perhaps ask the couple race teams that post on the forums that run track spec 370Z's.