@owenhooker, current there isn't an option to trun off selection highlight (preview) in Fusion 360. How important of this issue in your workflow? I feel sometimes this selection highlight is quite useful to tell me whether an edge or face will be selcted when click mouse. There are two options, not very good for this case, but hope it could help when you need to.
1. Unselect some selection filters, so less selection highlights, but it also disable selections, so not exactly what you need

2. Modify the environment settings (which configure highlight colors as background color so "no highlight/preview" in most cases). I don't recomment this way unless you need it badly. Because this requires manualy edit resouce xml files, and might get overwrriten when update to new Fusion builds. Please share us more info or detailed cases when require this function, it helps us prioritize issues. Thanks for sharing with us!
Lori Zhang (Fusion Development)