From the way that the drivers are set with Canon, it will require the edge up to a certain length unfortunately. Canon is aware of this on their end, but may not be much room to adjust it on how the printer is made.
Especially with sheets, this won't be an easy adjustment like rolls where you can do border or borderless printing.
For most thicker media, you would want to use the fine art tray feed method (as you would with poster boards and what not). Select the highest platen gap and choose the thickest media setting on the P800 for it to work properly.
With Canon, as long as you select the media setting, it should adjust the head space; otherwise you go into the settings of the Pro-2000 and change the platen gap (head) height directly. As long as it's a bendable media, it will work on the Canon.