There is at least one review on youtube that compares the performance of the PL-880 and S-8800 for sideband reception. That review seems to say that the S-8800 is marginally better, although it seems to me that some of that difference appears to be due to the positions the radios were in during the testing (as the reviewer points out near the end of the video). Whether the difference in performance is worth the increased cost is of course up to you.
As for the PL-660 and PL-880 sideband performance comparison, I have only a PL-880. I have a newer PL-880 and I don't believe my radio demonstrates any of the performance problems on SSB that were evident in some of the earlier units. I believe the word on the street regarding the PL-660's alleged superiority on SSB might have been valid earlier, but I would be surprised if the PL-660 outperforms my PL-880.