The gears on the XFire are a bit too close together, I would be afraid to dump from 6th to 3rd at 80, especially since it won't give you much more power than dropping to 4th since the peak power drops off after 5700 RPMs. If you are careful, it's very doable, just make sure you are not really already past 80MPH. You have about 1.5 seconds before shifting into 4th anyways though.

Now in the Viper, 3rd gear goes up to about 120MPH .