the stiffness must make a difference when you're hoofing a sprint
If it actually made any difference, you wouldn't see square taper on pretty much all the bikes at the World and Olympic championships. Subjectively, oversize axle feels much better, but Team GB, AIS, KNWU, FFC etc have big research budgets and haven't ditched their square taper chainsets, so there can't be any worthwhile objective performance gain from going big.
If it actually made any difference, you wouldn't see square taper on pretty much all the bikes at the World and Olympic championships. Subjectively, oversize axle feels much better, but Team GB, AIS, KNWU, FFC etc have big research budgets and haven't ditched their square taper chainsets, so there can't be any worthwhile objective performance gain from going big.