The GD series STi 6 speed transmission was a god-send vs the previous 5 speed. But, Subaru still left room for improvement. In mid 2006,they changed the synchro design to a carbon based one, which offers significantly increased strength vs the earlier versions. These deal with heat much better, making it a great upgrade for the road race guys.
We can offer a full package that changes all the 1-6 synchros to the updated type. Contact us at firstname.lastname@example.org for details.