How necessary is it to retune after a header, exhaust and lid install? Besides an extra gain in HP and turning off the CEL would it be absolutely necessary to keep the car running well??

 

This gets covered pretty frequently. The quick answer is about 95% of the time a tune isn't mandatory for exhaust and a lid. You might run kind of rich and your gas mileage will suffer if it's a DD, but you shouldn't really hurt anything. It just won't run the best it can and net the most gain until you get a tune.