Greddy Turbo TIC exhaust VS HKS Exhaust

[Znatic09]

I have plans in the future by the end of next summer to purchase a GTM turn key tt kit by the end of next summer. I am looking to get an exhaust. I have two in mind. And the reason why I am doing this is because I have had my Z for almost 7 months now and I have not done any modding yet. Very anxious to do something but do not want to be making mistakes on purchasing the corect mods for my big plan!,
1. Greddy Turbo TIC exhaust
2. HKS Hi power exhaust

So first, If I decide to purchase the Greddy and install with out the turbo kit yet, will I lose gain because of the piping size for flow (not very good with modding terms) or will it not affect it. If I decide to go with the HKS, will it be just fine with the GTM TT kit or will I need to upgrade?