It might light up. Never tried it. I try and use what's made for the equipment. That's a 600 watt dimmable ballast. You can get a Vivosun 600 watt bulb for $17. Same bulb I use.
If it starts then it won't run at 1000 watts and the spectrum likely won't be the same. The 1000 was made to operate at 1000 watts. Why would you want to run a 1000 watt bulb with a 600 watt ballast when you have a 600 watt bulb? The bulb can only use the amount of power put out by the ballast.
A 10-20% increase in output and shorter bulb life. I never use the setting myself. It's not designed to power a 1000 watt bulb with a 600 watt ballast. I'm not saying it won't light up the 1000 watt but that's not what that setting is for.