For a very long, very hilly with a time limit ride like PBP, I'd choose the fastest equipment available to me. Multispeed hubs are not as efficient as derailleur. The difference is small, but it's there.
A 2% efficiency difference doesn't mean that the bike is 2% slower, but it will be slightly slower particularly on climbs. Depending on whose Garmin it was, there are about 12k meters of climbing, though mostly on rolling hills.