If I set it to high, do I run the risk of getting false positives?
No more so that set to Normal as the sensitivity relates to what files are scanned and when, High basically scans more files on creation/modification, including those files not normally considered a risk. This can have a performance hit on your system depending on its specs, I have had the standard shield set to Normal for 5 years.
No, the false positives are not the problem here.
Normal is the best level balance between protection and protection. High could overkill your computer (in some configurations).