The 450 comes with the medium/hard pad and that's the one I would replace it with. In fact I've often wished there was a harder one to completely eliminate any chance of rounding edges over. The reason I say I would go for the medium/hard pad again is because it's easy enough to soften it by using the thick Mirka interface pad (not the thinner pad protector), but not vice versa, I.e. harden a softer pad.
I asked Metabo at a tool show last year what pads were available and I was informed that there was just the two types available. I wonder if another manufacturer's pad would fit.
I use abranet, and from what I read, the pad protector is recommended. I will get the medium hard one- I agree that it is a shame there isnt a super hard one, how I can see that it would prevent rounding over.