My hands on experience with this issue is only on my 987.2 2009 Gen 2 Boxster, so I'm not sure how relevant this will be to Gen 1 engines.
For months I had this very same issue, the white smoke is very alarming to have, I fully understand.
I changed my AOS twice (it is a different design to yours) and it is a bitch of a DIY job, a specialist is a good investment. An AOS is about £100, fitting about the same.
Neither of the AOS changes worked, still had white smoke but not on every run, just when there were LOTS of people around...
Following advice on here from a fellow 987.2 owner I dropped my oil level to one segment empty under Max arrow, so not 'Full'
The white smoke stopped.
A week ago, I had my oil changed by a much-respected specialist who I have used for years, and he filled the engine to one segment empty below max as per my instructions.
I have no dip stick, so just the crude dash indicator to go by.
Drove off down the road to massive white smoke eruption. Took it back, drained about a 0.5 L of oil out and all is well. Dropping the oil level one segment did the trick.
I have since found the dash indicator to be +/- one segment, so last check showed 'full' check before that showed one segment empty!
However, no white smoke.
Despite all this it could be the AOS as suggested, but before the specialist does that job (always a good maintenance action I think) have him drop the oil by 0.5L (no more) and try the car again.
If it smokes, then AOS change for you, or rather him, to do.
Good luck, I know how you feel, and let us know what the outcome is please.
Graham.
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