Not noticeably, I don't think. It'd be easier to just build a DIY generator or buy a CO2 duster for $4 and spray.
I worry about grow boxes being fire hazards enough without adding an actual fire into the mix. Have you looked into the yeast-sugar-water method of adding CO2?
Possibly, but as Zeddy said you don't want to be introducing a fire hazard. Plus, candles burn wax which gives off soot that can coat your plants and your grow box, definitely not a good idea.
i don't think we actually have a co2 issue...i was just thinking of something that could add a small boost with little effort. ah well.