A little more, but I think it's manageable, about 0.2W each 200MHz.
These measurements where the whole router consumption, not just the cpu!
CPU stressed running: time dd if=/dev/zero bs=1M count=1024 | md5sum
Idle: ~480mA on all frequencies, 5.8W
Stress 0.8GHz 26s (39.4MB/s) 500mA 6.1W
Stress 1.0GHz 21s (48.8MB/s) 520mA 6.3W
Stress 1.2GHz 18s (56.9MB/s) 535mA 6.5W
Stress 1.4GHz 16s (64.0MB/s) 550mA 6.7w
I attached a thermal probe to the heatsink with a bit of thermal paste, closed the case and ran it at full load for 20 minutes, I'm getting ~60ºC, so it's fine!
Perhabs it could get around 70-75ºC on a hot day, not dangerous temps, pretty comon for this devices.
You can always replace the heatsink with a larger one!