Are there performance implications using the "timestamp" property of ImportMulti in the following way? Based on the Developer Reference for importmulti, it seems the the timestamp property is purely used to determine how far back to rescan. If I have, say, 100 addresses and I simply set all of their timestamps to the same value (the earliest known timestamp), does this have any performance impact compared to setting each address' timestamp uniquely?

If the answer is no, then I'm wondering why this property was implemented as opposed to having a 3rd parameter holding a single "timestamp" for the entire list of addresses.
importmulti finds the earliest of the timestamps, and rescans from that point after all imports are done, yes. The wallet stores metadata about keys including the timestamp, which is why a timestamp is provided individually for each address rather than a single timestamp overall. Other commands like dumpwallet export a timestamp per key as well so this makes sense to provide it per-key when importing too, to save you having to look for the earliest before passing them in.