Yep the word cancelling is still being bounded about! The ECU keeps the car within its set tollerences and original capabilities. It appears the early JDM cars had a different ECU that has wider tollerences so freeing up airflow etc the results are not pegged back....
Manifolds do help with the airflow side of things as they allow equal flow (and a much nicer noise!) as the stocks are about 40/35/25 but then you need to look at the bore of the cat/hiflow/decat pipes y-pipe and then zorst as any changes in the internal diamaters are going to restrict airflow and ruin any gains that you have from the manifolds.
Yep, there not cheap, yes they are a PITA to fit, but if your carrying on with mods every little helps.. If you do fit / get fitted then make sure you heatwrap. Also if your going for a piggyback / remap then they will be benificial...
From research decats and true dual zorsts (with appropriate air flow balancing) seem to be a good way forward, if I was looking at a new zorst system thats the way I would go.. Go with hiflows if you cant be arsed to change them over at MOT. you just need to be careful when it comes to tuning with things like the AFR... mine was touch and go on getting a green light on the 10 second emmisions test.
Have a look back through posts by Adam from Z1 as he lists his modifications and tuning v his gains..
Also if your going down this route its advisable to get a baseline dyno to see whay your car is doing before you start fitting zorsts as this will give you an indication of what your car is doing now and what effects mods are making...