I found SME and any other clamp system tends to raise the LP off the platter a little because that's the natural tendency of pressing in the middle (unless you only do it a tiny amount, which never helps warps and doesn't couple more). In fact when it comes to any TT like an SME I always forgo the clamp when I'm the one placing the LP.
The stainless mat is very smooth. If you're a hot swapper you don't have to worry.
I haven't tried adding felt or leather. It should be noted early AS2000's had flat platters and people were sometimes trying leather as it was noted to sound good by manufacturer, but the latest platter (which every original purchaser got the updated one I think as a courtesy) included a slope. I believe they get a very nice weight with purchase, too. Were I try try something I might get some rubber to place between the platter and mat if I can find some really thin stuff, or it'd have to be pretty hard. Movement is the enemy between the LP and platter I think, which explains why a lot of the damping items sound meh (they move more but bring things to rest). The best sounding rubber mats AFAIK are not very compliant. The compliant ones I've had always sounded markedly better with cork or anything else on top of them.
