Engine performance all boils down to only 3 things: air, fuel, and spark. When it comes to the PCM, you're down to just fuel and spark (air flow is mainly hardware design, though VCT has changed that to some extent as well in the 2003+). So you can get better performance by running closer to MBT (minimum for best torque) spark or LBT (lean best torque) fuelling. The LS already runs LBT fuel (approximately 12:1 air/fuel ratio, compared to the normal stoichiometric 14.6:1) at wide open throttle (or wide open pedal for 2003+). Spark is normally the minimum of MBT and "borderline" (the spark level at which knock begins occurring). This is normally the only place where there's room for improvement from the PCM. But running closer to MBT when you're borderline limited would mean running higher octane fuel. But the LS already runs on premium, so you're going to need to find some really good fuel.
For most aftermarket tuning - all they really provide is additional spark advance. That's why they normally say to run premium fuel with the PCM change. But they were original designed for regular fuel, so the fuel upgrade option is much easier. This is true for most modern gas engines: the only real performance improvement is from more spark, which requires better fuel.
