ok i am confused. i have a 2002 z06. installed nitrous and my tuner said the car was running lean(without nitrous). before the tune he did, i made like 386whp(have headers and intake). now after he tuned it its only 363 whp
could it be reading lean because it seems i have an exhaust leak(cabin smells like fumes when i sit at the traffic light and when i shift up or down the exhaust pops like crazy)?

 

The exhaust smell in the cabin would mean running rich.

 

You can make more power on the bleeding edge of "safe" AFR right before everything goes boom. I know nothing about your before or after tune but I would guess that either the previous tuner was less concerned about your motor's longevity or your new tuner is more conservative... or a little of both. Running leaner than some would consider safe can make more power but could leave you with no margin for error, wear, sensor/fuel/weather fluctuations, etc.

Different dyno/conditions or same?