According to the ATmega data sheet, of the timers, ONLY Timer2 can wake the processor from SLEEP_MODE_PWR_SAVE and only when Timer2 is running off an asynchronous clock (probably since the I/O clock is off during Power Save). The Timer2 asynchronous clock pins (TOSC1 and TOSC2) share the same pins as the external crystal clock (XTAL1 and XTAL2, a.k.a. PB6 and PB7). TOSC1 and TOSC2 can only be used when the ATmega is running off the internal 8MHz RC clock.