It seems that ATMEL do not want to limit the AVR branch with 20MIPS AVR microcontrollers. Recently they introduced new AVR microcontroller family – XMEGA – which is still based on AVR core but has a higher performance along with low power feature. AVR microcontrollers are really optimal where high performance is needed with relatively low code size. Probably XMEGA microcontrollers will fill the gap between standard AVRs and AVR32 microcontrollers.
Better performance with AVR XMEGA microcontrollers - [Link]