I have a 97 z28 5.7l with 41k original miles. its all stock, with flowmaster cat back exhaust and k&n Cold air intake. If I get it tuned what kind of horsepower gains can I really expect?

 

In all likelihood, no more than 5hp.

Don't bother with a tune unless you've done significant modifications such as headers, heads, cam, etc.

 

Most of the tuners can net you 10-15HP by leaning out the overly rich target A/F ratio in power enrichment (PE) mode - roughly "wide open throttle". Stock tuning typically targets an A/F ratio of 11.7:1. You'll make more HP with a leaner target closer to 13.0:1.

 

on a note that is unrelated to your question...

That flowmaster exhaust will hurt you if your going for horsepower gains, It's one of the worst flowing aftermarket catbacks for these cars.

 

If it's one of the Flowmaster catbacks with the 3" inlet tubes then I'd just swap the muffler. It's supposed to be very restrictive and will hurt your top end because it's more of a stock replacement than performance upgrade.

I wouldn't bother tuning the car now if you have more mods planned for the future. It's a waste of money.