The zed like many modern cars is tuned to close to optimum from the factory. Any mods are going to be really chucking good money after bad, it aint like the days of ripping a single carb off a pinto and sticking on webbers or 4 socks.
Some cars though will take to modding, those that come with factory turbos for instance are more rewarding to mod over a n/a apication. That said the GM LSx range come over fueled for long distance reliability in the outback and to remove any undue stresses whilst still giving great performance. As a comparison to the Zed I just had my exhaust and headers replaced (equiv to the Zed exhaust and cat by pass really) and the difference is not to be soffed at. I have gone from 333bhp measured to 385bhp but the main difference is the torque has raised to 395ft/lb. Thats from the system and a remap.
For the same price the Zed will still chuck out pretty much stock figures because it comes in a box already close to optimum tune.
Personally, on teh zed, I wouldnt consider it on an NA.
On a further note, on LSx tuning the remaps are pretty much guaranteed and even backed by main dealer warranty because they are that sure of the work and reliability. IT speaks volumes you cant get that for a zed remap.