I agree...a 400 is your standard HID...and Ive seen some good bud grown with justthose. So a 600 will do you justice if you can keep it cool. Your setup sounds similar to mine.

I have a 2x18ft 2 door closet. I spilt it in half with panda film for veg and flower sides. Veg side is 400W Mh and flower is 400W HPS.

Keep it cool and you'll love your 600.