Straight from overkills sight under n/a pcm tunes
"87 vs 91+ octane The 3800 V6 engine is modestly suceptible to engine knock/ping/preignition when compared to its peers. If you're purchasing a tune and looking for the most horsepower, we can do the most for you if you're willing to run 91+ octane. If you're purchasing for a noticeable performance increase during daily driving, improved transmission life and behaviour, and fuel economy, the 87 octane tune will provide the benefits you're looking for. With breathing modifications such as exhaust work including downpipe, an intake system and ZZP HV3/rockers, 91+ octane benefits are available with 87 octane. "