Wow, this thread really got going
We do our best to check the forums but we're not on here all the time, checking every post. We just try to help when we can. Sometimes we're more active than others depending on how busy we are.
18000rpm, we don't always know who is who on the forum but honestly, you may have had a problem with your installer. We hate to say that especially if it it turns out to not be true but if it was who we think it may have been we did have a few complaints and when we looked into an install with at least one customer in detail, we did find issues that would affect performance of the kit. With that said, we're not wanting to call that installer out publicly or anything, mistakes happen, one day you have a good tech at the shop, a few months later, maybe the same talent is no longer with the shop and you get a different install tech just trying to do his best. It's a challenge we see that faces many shops, even the best ones. So we simply no longer recommend them and we'll just leave it at that.
All this really means is that we're honestly not sure that you got the same results as you should have on your GT-S. With that said, would that additional difference have helped enough for you to report different, it's really hard to say. Some people appreciate that difference more than others. Of course, it's still possible that yours was installed well but we seem to remember questioning the install at the time we discussed and never really got to the bottom of it but later found an error with the same installer that revealed a problem that was correctable (again, if we aren't mixing up customers here)
It's true that we don't quote any hard fast improvement times, most people with the upgrade agree that it makes a real improvement on the car and are very happy with it. Some owners install them on cars with an already low performing F1 system that has problems while another may install the kit on a well setup and maintained F1 car, honestly their results will vary a bit just for that reason alone.
Sport Cats, ECU Tune make a nice improvement but if you're talking about a big power adder, which we think of as 75+ horsepower, the only way forward is forced induction or perhaps NOS as JLobo has done. For the new Turbo cars, we can obviously do a bit more but there are limits as to what can be done on a Normally Aspirated car.
Hope that helps but we think the main point is that good improvements can be made to these cars not just in power but in response, handling braking, etc... but you want to have the right expectation that it's not going to completely change the car into a GT3 slaying monster or anything. These upgrades will enhance what you have and help you get more enjoyment from it.