I now own both(imroved eBay grip which I got for 60'ish) and an original Pentax grip(Thanks to Nick!).
And so here are some quick and dirty observations of both.
1. the leather material on the original is much nicer. Conforming to the curves and detail of the K20 rather than the geometric contour or pattern as seen in the above image.
2. the finish material on the original Pentax grip feels and looks different from the alternate(sheen and speckle size), which doesn't really affect it, though they are different in that respect. If I had to guess, I'd say the differences are in the materials and application. Though it really does feel as though the Pentax has the better finish of the two as it matches the K20D perfectly whereas the substitute does not.
3. the buttons on the original have a much higher quality feel to them than the eBay version. This includes the wheels and green accent which match the original K20D controls. The alternate grip doesn't have any green around the button(not a big deal, though it does seem to matter
).
4. the battery tray and assembly seems better built(tad heavier) in the original than the substitute version.
That pretty much sums it up. Installed they both seem to fit the K20D the same(tightly and solid), however, the original does feel better in terms of form and finish when holding it. I suspect this has something to do with the leather trim and materials used in each one.
Don't know about weather proofing.
I beleive the substitute grip adheres to the K20D standards in terms of weather proofing, though I can't quite remember if it was my particular unit or another one(shrugs).