We have up-graded alot of drives with Max Marine, shafts, treated gears, full towers, billet alloy top caps, have ran 900+HP with HI torque, what fails is the junk gears that you have to use, because of the way they are made. If you can keep the drive cool that helps alot, I block the water inlet and add a #8an to the left side of the drive and run the water to the top of the drive with a full plate drive shower that floods the upper with out having the water splash off the drive. The lowers hold up pretty well with a 1.250" prop shaft and a larger vertical shaft from Max, or Imco. I have seen the upper gears seperate from the cone clutch area of the gear, like you just twisted it off! Push out the spanner nut from the case, if it's not an alloy deep thread style from either Max or Imco. The upper gear back-lash settings on a good gear set, stay about right at .005 difference on total run-out of .010- .015 backlash others that are bad can be .008 on one side and 180 degrees be .018-.020. I would agree that 650-700 on an XR will hold up with out alot of hammering. Sorry, if that was to long.