I've just been through this myself. See the X51 thread below.

Apparently the only difference between the 3.8L engines of pre-Aug05 and post-Aug05 are a difference in Intermediary shafts. This should not affect the application of X51.

So although unofficially Porsche say that putting the X51 on ANY Carrera S should in theory be fine, they cannot put that into writing as the powerkit was released after Aug05 thus no testing was done on engines before this.

Adding the X51 would therefore invalidate both the X51 2 year warrantee and also your cars warrantee.