I'll let experts chime in here, here is what I think w/ what I know so far.
the 911 is a rear mount, while the cayman/boxster are mid mounts - making the engine orientation (facement) different - so I would say that the exhaust won't fit the 911 you listed.
The Boxster is an older generation one, but it does share the same size engine - so there is a possibility (and this is were I'll let others chime in better). I would guess it may be better to sell this one and get one specifically for that car.
Edit: My uneducated guess on the 2002 Boxster would be no, but this is just a guess and let others overrule it...