Thanks guys, consensus is that it's worth fitting so I'll have it done. I'm at 70,000km and it's been logbook serviced since new so not sure that that's included a trans service. I'll ask the gearbox workshop what they recommend.
It's not my first trip across the Nullabor and I've experienced the roaring headwinds. Last trip was a fair while ago - 2009 - when we moved to Perth and did the trip in 30 hours in a 1.3L Suzuki Sierra with myself, the wife, two staffies and a 5x3 trailer. Stopped for a 2 hour kip near Eucla and the winds were buffeting the car around even while stopped.
Was a surprisingly comfortable trip. Sierras have a great seating position and the 4x driving lights are all 55W HID - I was less uncomfortable at the end than prior trips in a VQ Caprice.