I had straight cut syncro engagement 1st-4th in my GC8.
you get over the noise pretty quickly, especially when your doing around 60k's an hour and you have to sit in 4th. in my opinion 6 speed is the way to go, and it will probably be cheaper to fit a 6 speed as well!
i had receipts for my one that was done at maximum and from memory was about 8k