There is a thread going about this over on Boxsterregister.org, I believe somebody talked with someone at Porsche and got the following guidelines:
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Removal of these parts [The big diamond shaped plate] can affect your suspension alignment. Porsche recommends the following steps be followed to preserve the alignment during this procedure.
- All wheels must be lifted and freely suspended without weight on them
- The vehicle must be supported as flat as possible using the recommended jacking points
Suspension re-alignment is not necessary if the support plate and diagonal braces are removed and installed under these conditions.
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At least one of the threads was for 987. I believe registration is required but here are the threads:
PCA Boxster Register
PCA Boxster Register
I just looked in the Boxster Service manual for the 986 Boxster in the section "Removing and installing the sheetmetal brace" and it has those same steps listed. It also seems to suggest that if these steps are not followed, the rear alignment may change such that the steering wheel position may be different than before when driving straight ahead.