I have yet to confirm this, but I believe it is a result of the high WOT air to fuel ratios that the factory PCM and injectors run. This results in much higher combustion temps and likely cooks the exhaust componants (like the converters) until they give off that smell.

Now that I have the TVT250 kit and am running lower air to fuel ratios, I will pay attention to it's effect on that horrible smell.

FYI... I do NOT have a stock muffler and the smell is still there.