Setting the U2X0 bit high in the UCSR0A registers half's the divider so it goes from 16 to 8, so you can get 2Mbps
But the arduino serial library handles that already by default, so you could do Serial.begin(2000000) from the start. (I have never tested it)
The atmel datasheet even states 0,0% error rate on 2Mbps @ 16MHz so that's nice as well.