The max temp you want with Co2 is about 90degrees. You are already at 85degrees, so you will not be able to use Co2.
Get your current garden in tune before you go changing the lights. 8oz from a 600watt set up is not very good. I was getting that with 400watts. Now that I have switched a 600watt system, I pull about 12oz every 60days.
Do some more research, setup your current grow correctly, and you will have a good harvest with 600watts. You mentioned you have a res, so I'm guessing you're running hydro. If you air temps are 85degrees, your res temps are probably right around that same temp. This is too hot and is keeping the DO out of your res. Cool your res, cool your cab, and you should be fine.
The light is the last thing you need to worry about. An experienced grower can out yeilds a newbie with half the light just because they know what they are doing. I have said it a million times, you can't buy experience.