I guess the real answer to this question will come from results of your grow. 85 starts to get near a line I personally dont like to get near. @ 84 is when my cooling kicks in. High 70's to low 80's is best. CO2 will always be a benefit. While there might not be an economic reason, CO2 will always be a benefit to your plants.
I find when supplementing co2 properly u shud be running ambient temperatures when the lights are on around the mid to upper 80s and at night mid to lower 70s. Having a window of 15 degrees between day and night temps is ideal.
When i say running co2 properly I am referring too. Making sure the rooms is sealed. Having constant co2 holding the ppm at constant.
Most importantly the co2 ppm should match the ppm of your feeding regime. As flower increases and decreases so should the co2 levels.
As co2 pom and feeding go hand in hand. Like workin out at the gym the more work and stress the more food intake u need to ingest in order to maximize growth benefits.
Co2 is great when run properly. But it can also have adverse affects on ur plants health if ur not running it correctly and can be detrimental to plant health.
Many new growers or inexperienced growers when using co2 find that they experience deficiencies and problems when they supplement co2. This is due in part because plants metabolism and food consumption increases w co2 bcuz u r changing the plants metabolic rate of growth. Like taking steroids is my example. When ur working out in over drive u need to increase ur protein, vitamin and water intake so u can benefit from the workouts and not be dehydrated. Same goes for the plants. If ur not increasing the base nutrients when the plants r in that overdrive state thats when u will get issues.
BOL