It is difficult to say how much longer the 316 stainless steel bolts would last versus the 18-8s. As you have implied, 18-8 stainless steel is less corrosion resistant than 316 stainless steel, primarily because it does not contain molybdenum. Most "rusting" of austenitic stainless steel is caused by exogenous iron (embedded free iron) that is "contaminating" the surface. Sources are numerous and one common one is using carbon steel tools on stainless steel parts. There are ASTM methods to remove free iron or you could try using a metal cleaner designed for this purpose. If "rusting" eventually occurs, depending on the severity it may only be a cosmetic issue.