[Lucky]

Sometimes friction modifiers make shifting worse. The friction modifiers in most gear lubes are in there for limited-slip differentials... so they don't grab and/or wear out prematurely. Your Boxster doesn't have an LSD.

The problem you (most likely) have is worn synchros which means two things: 1) the surfaces of the synchros have been polished by wear and 2) they engage a bit later (and with less pressure) since they have worn.

I suggest you start with a tranny fluid (e.g. Redline 75W90NS) that does not contain friction modifiers. See how it works for a couple of hundred miles. If you still aren't happy, you can add a friction modifier (without having to change the tranny fluid) and see if that improves the situation.

Good luck and report back so others can learn from your experience!