I still argue for elimination.
As me1 and I pointed out, there is the meaning of accomplishing something with a dramatic, forceful nature, and PONS actually lists "slam dunk a proposal" http://en.pons.eu/english-german/slam
My original point, though, is that the phrase isn't an idiom that people are going to be looking up. I find almost no hint of it on the internet, though there are 37 hits on Google this morning, the top two being this discussion. The new meaning, if it really exists independent of questionable dictionary entries, is so new that it's scarcely represented even online.
Frankly, the AHD citation for "defeat or dismiss decisively" doesn't strike me as a particularly strong example. What is it supposed to mean? They've stopped defeating each other decisively and begun designing strategies with the aim of moving those on welfare into decent jobs? Or maybe, they've stopped dismissing each other decisively and begun designing strategies? Maybe if we had the rest of the context, we'd know.
I'm not opposed to entering the "new" AE meaning of slam dunk
as a verb, assuming we have more examples and reputable sources, but I still oppose "slam dunk a proposal" because the meaning isn't dependent upon the "proposal" part. It's not an idiom, as far as I can tell.