If I remember correctly, a P80 had more pronounced inlets for the engine. They protruded into the airstream considerably more than a T33.
The stem will pull some heat from the valve face and transfer it into the valve guide and head. Sodium filled valve stems have been used to help dissipate even more heat. They have drawbacks as well.
When a valve discolors like in the pic, it is usually due to a failed valve rotator. Debris builds up, causing the valve to dissipate less heat into the valve seat and head. The valve begins to warp and it dissipates even less heat to the seat. It just keeps getting worse for the poor exhaust valve