Typically cars with very good flowing exhausts are more likely to exhibit it, we saw this mostly on the original P17 housing, If recall all the vf39's now ship with P18 housings (I could be mistaken here) which fair much better.
If you have a turbo in your hand I would port the thing anyway, it's not very difficult to do and for the little time and effort can lead to better spool up as well as protect against the possibility of creep.
Mike
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