Definitely go for the GX-5R in your truggy with the 2072 pipe. It will kick butt.
The GX-5R is dominating the truggy class here. If you don't get around 8 gallons or more out the 5R with proper maintenence I would be very surprised.
Visit our website at www.go-racing.co.nz and go to the Tech Talk page from the menu. There you will find break-in, tuning, and on going maintenece tips to ensure you get the best performance and longevity from your new GO motor.
Once it's done a gallon, there are a couple of things you can do to increase power and economy further. Just PM me if you need any further info.
I am not trying to sell you a motor, as you are better off buying from the US in your case, just offering advice to make sure you enjoy your new motor and pass on the good word to others