Spark plug indexing is indeed a good 'ol racer's trick, but:
- It's much less useful on engines with centrally-located plugs (i.e. go down through the valve cover) than side-entry plugs.
- There are more gains to be had by ensuring the plugs are gapped properly. It appears the gap wasn't checked in this instance. As has already been mentioned in the thread, a plug can have multiple applications and it's likely not all of them have the same gap.