Wouldn’t it be great is you could also use the ’16u2 on the Uno or Mega for some additional functionality to your Arduino sketch ... between the ‘328 ISP header and the pins on the ...

It’s a significant update. The Arduino UNO and Arduino Mega 2560 update the Duemilanove and Mega respectively. They now use an ATMega8U2 instead of an FTDI USB to serial converter chip.