The current Vision-3 stocks are designed to be intercuttable so have a very similar gamma and saturation between them, the main difference just being graininess due to speed. However, there is some belief that a fast stock has a lightly wider exposure latitude because it has fast and slow layers for each color record compared to the 50 ASA stock. Not sure that translates into lower contrast but it may allow you to pull a little more detail in the far ends of the scan. Or it may just be a wive's tale... though I sometimes feel that 500 ASA stock outdoors has a somewhat "flattened" feel to it.
Overexposure + normal processing won't lower contrast but obviously it records more shadow detail at the expense of highlight detail just as with a digital sensor. However due to the characteristic curve and how overexposing burns detail out slowly to white, often there is a greater need to expose for shadow information than to worry about the highlights. Film negative basically likes exposure. But if your sunny high-contrast scene is mostly made of bright highlights (snowy landscape, backlit ocean, wedding dress in the sun), then overexposing may not be the best solution.
Overexposure + pull-processing does lower contrast, flattens the characteristic curve a little.