Update and verdict:
On Saturday I replaced my Made in Mexico repro seat pad with the genuine MB article and drove the car on Sunday. The improvement was immediately noticable, and after an hour of driving I was still pretty comfortable. With the repro seat pad I'd be feeling a fair amount of discomfort in an hour.
My conclusion is to avoid the repro pads, which seem to be what many of the aftermarket parts suppliers are selling in the US. Get the MB pads. I got mine from SLS in Germany, but an MB dealership or Tom Hanson might be better choices in the US to avoid the shipping charge.
Regarding some of the other discussions:
I think it would be hard to make your own seat pad out of some other material, though not necessarily impossible if you're determined and have the right tools for cutting and shaping foam. If you try this, you want a pretty firm foam to match the firmness of the original horsehair. Getting the shape and thickness right will be difficult, and it's probably better to just buy the correct pad.
To clarify terminology, I think the terms "seat pad", "horsehair pad", "rubberized pad" all refer to the same thing. The pad looks like a bunch of stringy material (like horsehair) that has been coated with rubber, packed together, and formed into the proper shape. Don't know if horsehair is really used, that term may just be a holdover from the good olde days.
Hope this is useful to someone!
George Davis
'69 280 SL Euro manual