Yes, the more light the better. But you should get adequate lighting with the 400 watt hid for that space. Just the same, if you can keep it cool, try for as many watts as possible.
If you find that you have heat issues with a higher wattage hid, you can always supplement with fluorescent lights. Fluoros burn cooler than hids. Are you able to manage the heat with what you're using now? Do you have good ventilation? If so, try 600 - 1000 watts and see what happens?