My LT4 is stock... stock headers, factory cats, stock camaro airbox (with K&N). It easily makes over 600 rwhp. The tune is a blend of '16 CTS-V and '17 ZL1 with the fueling dialed in perfectly (stock AFRs). I did a couple dyno pulls with it leaned out a little and it easily made 620 hp and 650 ft lbs (SAE). I dialed it back to stock AFRs running less timing than a ZL1 and made 605/633.