First off, our ignition systems use what's known as a COP setup, or, coil on plug.
Our cars don't use plug wires, this is because individual ignition coils are connected directly to each spark plug. no plug wires, and a direct connection to the coils makes for an excellent ignition system, better than anything the aftermarket offers. the whole reason toyota, and many others, chose to go with this setup is it's far more efficient and reliable. no spark plug wires means no extra resistance (something which almost all spark plug wires have)
On a conventional system, with plug wires, even if you add some nice fat racing plug wires (like the 9MM jacobs racing wires I installed on my escort) you will still get plenty of resistance from the wires. This is because highly conductive wire, like copper core wires, produce lots of interference and signal bleed, which will produce unwanted noise through your stereo system, not to mention screw with all sorts of electronic devices like the ecu, pacemakers, etc. this is why even racing wires have lots of shielding and resistance to cut down on unwanted interference.
While some race cars have used copper race wire with good results, the interference that is produced with them makes it illegal to use them on the street or in most sanctioned forms of racing.
by putting the coil directly on the plug, you get the benefits of no resistance with none of the signal bleed. This is why companies like SplitFire make aftermarket conversion kits for cars like the 240SX, which use a remote-coil ignition system.
The GTS for certain come with Denso Iridium plugs from the factory, not sure about the GT, but I think it may come with them from the factory too.
The only other thing I could reccommend is to go with an ignition amplifier of some sort, or new, high-performance coils (which aren't available as far as I know), but seeing as the coil is already on the plug, an increase in coil Performance may not make that much of a difference, if anything, it'll make your spark too hot.
If you run nitrous oxide or Turbo, you need a cooler plug, or else there will be detonation. Switching to nitrous plugs, or platinum multi-electrode plugs will help. Stay away from copper and irridium plugs on FI/Nitrous applications, as the plugs retain too much heat, and will set off the A/F mixture before it sparks (pre-detonation).
Copper plugs are the worst, in our engines, they can get so hot that the electrodes can melt!!! don't mess with copper plugs unless you know what you're doing, and intend to tear down the engine on a regular basis (think dedicated track car)