1) I like the Shipibo pads on my RS1x for the same reason. Switching to fully over the ear pads helps to seal in those deep bass juices and tames the highs a bit. For most Grado models this is an appropriate adjustment.
2) Pad swapping is a fundamental part of the Grado experience. Tally up what % of the posts on here are about pads. Some folks are going to tell you that you're "killing the Grado sound!!!!" And some folks (like me) are going to agree with your preferences.
This was a great post. If I could give it more than a "like", I would.
3) Why Grado does what it does is one of those mysteries of the universe that we may never know . . . like what happens after we die or why people listen to Nickelback. They sell tens of thousands of headphones every year and keep us talking about them 24-7 so they're onto something.
4) I dont think you're going to notice much difference with any of the Prestige Series models, at least I dont. But they're cheap enough . . . so why not collect them all!