Any leakage at all and you get additional noise and loss of low frequencies. I would recommend that you do not change any of the others unless there is an actual fault; there is no advantage.
On the grounding, there should be a ‘shakeproof’ washer next to the paintwork that will cut through the paint and make a good ground contact. If there is not a good contact then it’s a good idea to clean away some paint…. it is essential that the ground contact is made. Yes the chassis should be grounded at pin 1.
That solder joint is correct.
As for solder changing the sound….. No! it’s your imagination! I use lead solder wherever possible because it’s easy to work with, but there cannot be any difference in sound. It’s like loudspeaker cables, weight is the important thing, not uni-directional or oxygen free. 60 amp cooker cable is best!
I’m glad you are enjoying the P3…. it’s a fine mic amp and the compressor is a genuine optical compressor with a superb sound.