My 10c worth
In my humble experience flying Metro 11 & 111s.
Fuel balancing is a procedure best left to the refueller, or if the fuel order has been stuffed up, done while taxiing. A swift turn on the ground with the high tank on the inside of the turn and the fuel Xflow valve open will shift a few hundred pounds quite quickly (it is a 2" pipe).
If you have read the manual, take off or landing is prohibited with an imbalance of > 500lbs (varying between S/N & weight). This may have prompted the NZ boyz to conduct the procedure in th first place. Though conducting a maneuvre such as a side slip with crossed controls was perhaps not wisest of decisions at 25000'. The Metros I flew didnt have a flight manual procedure for fuel balancing or auto pilot limitations, I guess as most of the auto pilots in Metros are retrofits.
I heard a story of a Merlin being flown with the cross flow open and the auto pilot on. Apparently, it ended up losing six thousand feet in a barrel roll before regaining control
One Metro I flew always transfered fuel to the left regardless of the fuel levels. Fuel could only be transfered the other way with the TB a whole ball to the right! No ammount of time in the workshop with the engineers could fix the problem. The condition was probably due to unbalanced efficiency of the fuel tank vents.
I dare say the best advice if you intend to fly a Metro or currently fly one is to:
1.Do the fuel balancing on the ground
2.Practice transfering fuel on a base check with your C&T capt or endorser (without the auto pilot engaged)
Blue skies
Jarule