I got those L-Pads on the Diamond tweeters and the volume is perfect. Still thinking about moving them to the doors though. they're still a little bright for my taste and I think it may improve imaging. I started a new thread on that to see if anyone else has tried it.
Are you sure that door skin is metal? It sounded like ABS or something when I tapped it. I'm curious about this as well. I'm tempted to cut it to get an absolutely perfect fit but I'm nervous about water getting to the speaker.
I really don't see how the 6.5" diamond driver could possibly fit perfectly without either mounting to the outside of the panel or shaving/cutting that door shell a bit.
As far as the spacer ring goes, I think that's only the HK speakers. I'm pretty sure the stock speakers have it molded into the speaker and the grille design.