The efficacy of synth oil depends on large part on the condition of the engine that it goes in. My local speed shop has seen noticeable power increases when an older car switches to synth (Royal Purple). This is confirmed on the shops dynojet.
The mechanics stated that although the IS would benefit from synth oil, it probably would not result in noticeable power increases given the new condition of the motor. But the shop people agree that as a whole synth oil has more positives in a motor than negatives.