Yes the steel sandwich plate versions are obviously stronger since the gearset is supported by a steel plate instead of an alloy plate. An alloy plate is more likely to flex under heavy torque loads; flex = misaligned gearset = shredded gears.
Wilbo and I compared a late W58 with an early W58 and we found a few minor differences - some of the bearings and gears are slightly wider and the synchros are a different design - but the main difference in terms of strength is the steel plate.
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