Use the SoftFever (Orca Slicer?) fork of Bambu Studio on GitHub. It has a calibration option on the top menu bar that will greatly assist in dialing in nozzle size for flow, PA, speed… Another thread in here about the 8mm nozzle. IIRC someone was able to increase the flow rate (mm3) and speed and cut the print time 30% or 40%.
You would be better off as Mr.Exocomp stated by increasing the flow mm/3 and speed rather than temps.
Using a calibration would be best but a WAG would be 50% more than the .4mm nozzle to start and see how it does.
I have yet to do mine so I can not give any baselines to start. Logic says the X1C and P1P should be able to do the same speeds on all nozzles, just need to get enough filament through the nozzle on the larger sizes to keep up. An increase in temp may be needed.
OrcaSlicer has several Calibrations that will let you max out and fine tune your printer for a .8 nozzle. He put them in proper order on the Calibrations tab at top. Don’t forget to look at his Wiki which explains how to use them, with pictures, it’s in my post.
I usually run a .6 and found after calibrations tests that I could run the Max Volumetric Flow almost three times the default value for PC (polycarbonate) and it tuned other settings for beautiful smooth surfaces as well.
Don’t forget the rule of speed: If you increase speed you must step up the nozzle temp too to help melt the plastic faster and get good layer adhesion. Which means you also must up the fans to cool it for each layer. This stuff gets complex quick! That can all be saved as a Filament Profile for future one click access. I name it like this: Polylite PC Red .6 with the nozzle size so I remember which profile is which.
The main problem people have is when they say a 0.8 nozzle is not speeding up the print or even taking longer is usually that they do not adjust the wall count and infill, so print the same model with double as thick walls.
But volumetric flow will limit the possible speed of a 0.8 nozzle quite a bit, and if you really want to push, get one of those aftermarket CHT clone nozzles for high flow. Currently they aren’t available in the right height in a hardened version though, but I guess its just a matter of time till that changes. Gives bout a 35-40% in max flow increase.
For larger models I am already hitting the default volumetric flow limits with the 0.4mm nozzle, I doubt going to 0.8 makes much sense with the default hotend, 0.6 makes more sense but only for certain prints.
I guess it depends mostly on what you are printing. If I were to print large parts with the same filament I would calibrate it for flow and temperature with a 0.6mm nozzle and increase inner wall and infill width to match what the hotend is capable of. I tried just bumping temperature and flow and for most of my filaments it didn’t work too well as the finish of the print showed the different flow/temperature difference too much (different matt/shine in places).
For faster printing you’d be better off with a clone hot end with a clone CHT brass nozzle installed. Original Bambu is OK for smaller nozzle but on larger sizes hotend melting capability becomes a bottleneck.