good luck with the mods, but i'll give my little opinion.
i KNOW it is subjective, so this is not a criticism of you, but it is another perspective, along the lines of sj156's sentiments:
*i dislike trying to make a car perform or look like something that it isn't. so, trying to make your non-GTA look like one just doesn't work for me
*be wary of bodykits made of fibreglass: they may fit poorly, but mostly they are easily 'cracked' if you nudge a curb or pole or whatever; this includes the bottom of the front bumper, and if you scrape a steep driveway repeatedly, it will damage the fibreglass; the results look terrible
so i vouch for the Zender kit. whilst it's not exactly like a GTA, it IS high quality, being made from flexible ABS plastic, like your original bumpers. the fit and finish is also excellent, reflected in its not-so-cheap cost. there are 3 choices of boot spoiler, including the high-wing, low deck spoiler, or small bib-spoiler. i had the full kit with the small bib spoiler on the boot. i thought it was great: it made the car look different in a nice way, without looking like a GTA.