Going from HKS Hi Power to GTM 3", what to expect?
 

Quick question:

I'm prepping for my stage 2 GTM TT kit that I'll be ordering in the coming months and thinking I should ditch the HKS for a 3" GTM exhaust.

I just got tuned two weeks ago, would it require a retune and how much power do you suppose I would lose?

I'd wait until you order the TT kit. You will need a retune if you hope to minimize power loss but will likely still lose a bit even with the tune. Its just too free flowing for a NA Z. I'm certainly not an expert so this is more of an educated guess.

you wont need a retune. get it now if you find a deal, otherwise no harm in waiting tbh

Tune won't really change much since it is on the tail end (literally). You'll lose some torque until you go FI.

^That's correct, you will lose some torque but likely won't lose much horsepower. I too would wait for your FI installation.