Effects of long tube headers?

LeaK

Hey everyone. So Ive had long tube headers installed with cold air intakes without a tune and without defoulers for a couple weeks now. Plan on getting a tune soon.

Obviously with all that it’s going to mess with everything but I’m hoping you guys can help ease my conscience and tell me what these mean.

With these mods my car is running lean but my gas mileage has dropped to about 15 mpg

As far as I can tell there are no exhaust manifold leaks. No ticking, tapping or other unusual noises but how can I verify there are no exhaust leaks?

What are the symptoms of an exhaust manifold leak?

At idle with the with the ac on my Z has a bouncy idle between 800 and I think 600 rpm.

I’ve read how the ecu can compensate for either full exhaust system or intake modifications but not both. Would you guys say it’d be best to put my stock intakes back on for now?