I still have the case open. They didn't have an explanation for this.
Our SE suggested we run one cycle of regular Fulls, and then return to the Syn-Fulls. We've now completed that, and no change. The Syn-Fulls on our file servers, that used to finish in approximately 36 hours are taking a very long time. I have fulls that were started at midnight Friday (10/14) that are only 26% done as of late morning 10/17. The Fulls that I ran the previous weekend would complete in about 36-40 hours. So I'm going to have to change all my file server backups to Fulls. I can't afford to let a Syn-Full run for 1 week.
I don't know what the explanation is but, there is a significant difference in the "average throughput" when I look at the backup job details. The syn-fulls run around 20-25 gb/hr, and the regulart fulls run at 50 to 80 gb/hr.