How to improve printing over 100 mm/s with 0.8 mm nozzle?
There are no ready solutions. Check on Klipper Github to see if someone requested that, but I doubt it has been implemented.
100 mm/s with 0.8 line width and something like 0.3 mm layer height results in 24 mm^3/s. It's quite high already, since the Volcano is rated at about 20 mm^3/s.
What you could try to do is to take the GCODE you have, calculate the layer time and the total path length for each layer to get the average extrusion rate, and introduce at the beginning of each layer a M104 command with a temperature dependent on said rate.
For example, you could add 1 °C for each mm^3/s of extrusion rate starting at 15 mm^3/s.
If you want, you could also calculate the average extrusion rate not every layer but every 50 mm of extrusion. In this case, better anticipate the temperature command by one stretch, since it takes time to adapt.
Otherwise you need a Supervolcano, or a slight overvoltage of the hotend (remember that power goes with the square of voltage, so do not go above 10% overvoltage).