I don't understand the question/scenario (unless V5 is different than V4). I only have the option to "turn on/off scan at startup".
I know of no option to do a 'new only' scan in Studio One 5. I'd love to have one though.
If I turn off scan and I add a new plugin and then re-start Studio One I don't get a scan and the new plugin is not found.
I do a full scan at startup, and in the course of a day I close and re-open Studio One with some 'reasonable' period of time, a full rescan runs much faster (~15 seconds) than if I close Studio One and don't reopen it until the next day. Then a full scan takes a fair amount of time (~1 minute).