Preloading does help quite a bit but wont do much if you are not slipping. Preloading will reduce any sudden shock against the drive line - sure it might hurt the clutch but a hurt clutch is better than a hurt gearbox - if your tires hook up hard and you have enough power to do any damage, the G series box will fall over like a house of cards. Seen it happen on far stronger boxes. Of course at most test and tunes the track is barely prepped so it wont generally be sticky enough to be a problem.
get out there and practice - ignore the tree while practicing...reaction time means nothing at a test and tune - the timer wont start until you roll the beams, so wait for the lights to count down, get relaxed, and concentrate on launching. Once it is second nature, start playing the game on the tree.