The Gas Monkey Dart actually had the Hellcat engine w/powerglide. The Roadkill Charger had the complete Hellcat drivetrain and lost the race if my memory serves me correctly.
I'm not familiar with the episode but wouldn't the Charger weigh massively more than the Dart?
Yes, even BEFORE they started modifying. The Dart went on more of a diet than General Mayhem did. Gas Monkey built their car to win the drag, Roadkill built Mayhem to have all sorts of fun.
That's sorta the whole point of the discussion- if you JUST want to win the quarter mile, more gears in the gearbox don't necessarily help. But if you want the car to do very well at the drags, cruise 85 on the toll road and get 20-something mpg, and run a decent road course all without swapping rear gearsets, changing convertor stall speeds, etc. then more gears works great- its more practical and flexible. The Powerglide for drags is a one-point optimization.