I'll throw in my 2 cents since I did get my Z uprev'd at Hills.
Expensive - yes. Worth it - YES.
You can take a look my list of mods in my sig line below; they're all bolt ons as well.
Everybody suggested I get tuned since I swapped out the intakes and replaced the entire exhaust system.
Apparently this was enough to make a difference.
What will Hills do?
Put your Z on a dyno, run it up to 4th gear and 7,500 rpms, take readings, bring it back down to idle, make some adjustments, repeat until satisfied.
Here are
some pics from my tune.
What are they doing?
Simplest terms - flashing the brain.
Apparently the Z has about 15 different "driving modes". "It" determines what mode it wants to use based upon how you drive.
What does the tune do? Gets rid of about 12 of them, and basically locks your Z in "performance" mode. So regardless of how you are driving, she's ready to respond.
Frank got about 20 extra horses for me. I love it!!!
hth