Look primarily at your camber settings. Even with default suspension settings the ZEXEL is as quick as most of the other JGTC's. Just give yourself a 2.4/1.4 setting as a baseline (it's a tiny bit more than default) then adjust from there.
Which championship are you trying to compete in anyway, if it's Japanese GT Championship with the other JGTC's, you can easily win the whole thing using the less powerful BMW M3 Race Car. 473hp after oil change, no other mods and better on the tyres than all the JGTC cars. And it gives a decent amount of points too after beating those cars.
That, and the JGTC in GT4 requires the car to be Japanese, which the M3 is not, Mafs.
Might also help you to try running 85/85 ride height and setting the spring rates equal to each other at around 8-10 kg/mm. But TEST it. If it doesn't work better, don't keep it. Also, I'd suggest more radical rear camber than Mafs did, so about 2.5 front and 2.0 rear.
Waiwaiwai.... You have issues with traction on a JGTC car enough to need 0 camber?
They have more downforce than DTM cars, and my DTM Audi TT (of only 20-50hp less than a JGTC car) has 1.8 and only spins the tires ever in first.