Pork fat starts melting 104*F beef fat starts melting 120*F but that doesn’t mean they leave the sausage at that temp. That said the Polish poach at 170*F but I tried that in SV and was a dumpster fire. In my smoker most of my sausage comes out after 4-5 hours at 120-130*F and into 151*F water bath for 30-45 minutes and they are done. I think the longer they are in the hot water the more potential for fat out, that’s just my experience.
Here is what Stanley Marianski has to say about fat and temperature.
”There is very little fat loss between 150-190*F or even up to the boiling point of water 212F. There is a significant fat loss at temperatures over 248F. This is the range that covers barbecuing, grilling or roasting. At those temperatures the fat leaks out of the meat”
Chapter 6 of Home Production of Quality Meats and Sausages, page #104.
What he doesn’t say is how long you can hold meats “in the correct temperature range” my experience is not long at all, but then I have smoked sausage for up to 10 hours ramping up slowly from 120-160F with some fat dripping but not bad at all and time above 150F was short. SV though seems more powerful (obviously) than dry heat, so again it comes back to time in temperature, keep that as short as possible. This all assumes a quality grind and no fat smear.