well the main cats are right by your tranny and stuff, so if they are hot (which they do get, friction even if its "air" generates heat) and hot air rises to it follows the chassis and goes to under your hood or just heating up your tranny some more.
Or something...

My precats are gutted (1st precat actually blew and was burning in my 2nd precat) and my main cat is... ghetto, its 1/2 gutted i guess since i didn't want to break the bar thats in the middle of it. Looking to replace all the stuff soon, if any of my junk sells. I get that smell of sweetness in the cabin too, to me it kinda smells like atf so i always just think its that leaking/overflowing (i never know how much fluid is in my poc tranny... blame badly designed L4N71B auto trannies... ) and vaporizing

 

I still was able to gut my whole main cat even with the air tube in the middle, What i did was took a skinny peice of Wood molding for some trim,. starting slamming it down the sides was able to get everything out, was not fun but i did it, My pre cats are still there i was thinking of taking a Drill with a Flexable shaft with some sort of thing at the end i can shove up there and get it out, Ill goto the store and see what they have there... but on the other side there was this plate thingy, i didnt have a cutter or a tool to take it off, so i could only get it out from one side!

 

Kouta pretty much sumed it up good. A down pipe is what you need. Properly gutting the cat is not worth the amount of trouble that goes into it (specialy the other cats were you can just weld in a pipe). To properly gut you cat you need to:

Take it off, cut it in half from the side (so it comes apart kinda like how a book opens). Remove AAAAAAAALL the stuff thats in there, and weld it back. That, is how you properly gut a cat. Its not worth all the hassle.

 

Nah, ramming a screwdriver in there to break up the bits works just fine. Then you poke at the sides a bit. DO NOT INHALE THE DUST. But the whole thing should cruble right out.

 

I got it all out with out cutting it apart, Nothing is in there at all. Just mashed it up and then turned it upside down, the stuff came out, **** Cutting the thing apart

 

Yeah, I think i am going to punch out the rest of my cat when i drop my exhaust to put on new gaskets since mine just disenegrated and now I have exhaust coming up everywhere. Even with cleaned out cats there is the turbulance issue you have to deal with now, but its still better flowing than having the converters actually there.