black smoke from exhaust under hard throttle

Well, its a good sign that its not white. Are you sure its black and not blue? If it is black, then it could in fact be carbon build up getting blown out. If its blue then youre burning oil.

Well maybe with those 38# injectors ...perhaps your running too rich.

The chip you have installed,maybe this could also be casuing it.Oh and,about the cat leak,where exactly is it leaking from? Is it before or after the cats? If its before,meaning between the headers and cats,it's possible,just depends on how bad the leak is.If it's right next to an O2 sensor then that could cause a problem.Could be picking up too much oxygen and then adding more fuel thinking it;s running too lean......just a thought.

I would agree with 5.0 because thats the color of burning oil. I think the carbon build-up would be more of a smell than a color..unless there were BLOCKS of it in there, which i doubt. The same issue came up with a buddy of mine who had a vortec on his LX. Before he got it tuned, you could see a faint black smoke coming out of those outrageously loud tails..
My other guess would be an issue with your cats and 02 sensors (well pretty much exactly what 5.0 said before) if all your other options have been exhausted (pun intended).

That smell your getting is coming from the exhaust leak.It's what the exhaust gases smell like before they run through the cats.That tells you right there just how much emmissions garbage gets reduced through the cats.

And if your going for long tube headers,the front set of cats I know for sure will have to be cut.And if you plan on removing them alltogether,your gonna be smelling a lot more of that gasey like exhaust.It wont be that bad if all the rest of your emmissions system components are in good working condition.Adding long tubes toys with the O2 sensors.And believe me,the O2's on cars with OBD-II PCM's and the fact there are 4 of them (O2's),these systems are more difficult to tune properly without having lean/rich errors coming from them.Remove even 2 of the cats and you have gases that play with the sensors because the computer still thinks all 4 cats are installed.This occurs due to the O2 sensors on back side.

Well,I have to shut up for now.Writing too much again......