Probably true as a general proposition, but I read occasional reviews of HD disc releases which refer to them as "train wrecks". Fifth Element for example, where I have specifically read statements that the SD disc was better. I have both discs, and I've never compared them, but I can say that the HD disc sucks. I think someone sneezed on the lens during the opening scene, so the disc is proof that people should cover their mouths.
The Sony Blu-Ray has gotten good reviews as an SD upconverter, but is that the best for an SXRD? Don't think so. Not for the price. And the freeze-ups. I think that part of the benefit of the Sony Blu-Ray player is its ability to output 24 frames/second instead of 60, without paying the price for a Pioneer. Though that may only be for Blu-Ray. And I don't think the SXRD can play 24 fps anyway.
Last time I checked, the SXRD is nice, but I don't think that it has any qualities which would uniquely match it to a particular upscaling SD player. So probably better answers to be found in threads addressing the best SD upscaler overall, without regard to the display model.