Be very careful with higher volumes else you destroy your hearing. There are special IEMs that can be customized to your ears that block outside sounds and can be used at low volumes.
I doubt Kawai publishes detailed specs about the headphone amp on your CN29. Maybe assume it is just average in everything. The "detailed" specs by Beyerdynamic, or any other audio company, are not very helpful. There is some fuzzy science matching amps and headphones. So just assume there is nothing perfect. In your shoes, I would just assume the 32 & 80 ohm rating headphones will not be so different.
All else equal, a headphone with a higher sensitivity rating could be louder but the measurements are not reliable so that doesn't help too much lol. Only way to know is to try.
You also have the option to buy a dedicated headphone amplifier. With more power, switchable impedance, better quality electronics.
Beyerdynamic has some clues on what the different ohm ratings mean. https://support.beyerdynamic.com/hc/en-us/articles/201058791-What-do-impedance-and-ohms-mean-