Easily 425 degrees. It is a wet enough dough that some of the leaving comes from steam. Are you making your own dough? It has such prolonged proofing periods that it benefits from a bit of honey in the dough. That will help with browning.

If you aren't making your own bread and thus cannot control what is in it (honey, oil, etc.), I would try browning it in a pan on the stovetop. It would be a cinch if you could add oil to help the browning process, but give the stovetop method a try. Boulangere's suggestion is right-on, as well. I find that people are skittish about setting their oven high enough for certain applications, and 425 is not too high for toasting bread.

Make a thin glaze by bringing to boil one tsp of flour in 1/2 cup of water. Brush the focaccia with the glaze before baking. It will produce a glossy brown crust.