Rigging up the Christmas lights and this year the tree is around 700W all incandescent.  I have a 600W Switchlink in a metal box connected to a cord plugged into the box (so the dimmer is in free space not in a wall.) I see the triac is rated for 10A.  Anyone know of a hard limitation to 600W.  If I can keep it cool enough, 700W should work??  That's "only" 6 amps.

Thoughts?

600W would be more of a heat limitation than just amperage through the triac. That's close, but would probably be ok if well ventilated. Just a personal hypothesis...  Will you be dimming it? Otherwise a relay switch would be better. 

Maybe add a box fan to the enclosure?

Depending on if the metal frame of the switch is mounted on a surface that will help remove the generated heat. Incandescent loads also may have a current surge when starting with a cold filament. To consider.

Relay type as pointed out would be better if you don't need dimming.

Even better would be to split up the load between two switches. Depending on what you have available, a few lamplincs could be plugged into a power strip. They're only rated for 300 watts though so you'd need 3 or more.