I have both radios but favor the FT-817 for SOTA expeditions when I choose to carry an all band radio. (I usually carry CW only radio that is much lighter, but carry the FT-817 on some trips). Clearly the out of the box filtering is better on the KX3, but so is the price. You can buy two 817's for the price of a KX3. However that said, I set out to modify my 817 to see if I could close the gap between the two. So I added the W4RT On Board Filter ($284) with both SSB and CW (300Hz) filters. I also added the BHI DSP filter ($169). These prices include installation by W4RT, if you do it yourself you can save a few bucks. These modifications, in my opinion help to close the gaps between the two radios considerably.
Below are some very basic, unscientific, comparisons of the two radios on SSB and CW.
As you heard in the videos the differences boil down to a matter of taste. I think the 817, with the 300Hz filter comes very close if not better than the KX3 on CW, on SSB the KX3 has the advantage but the DSP does clean up the noise and the SSB filter does sharpen the 817 audio considerably. I favor 817 for outdoor work and the KX3 for the in-shack QRP radio.
In future blogs I will discuss a couple cool additions to the 817.