SS vs XR8 and the winner is......

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Here's a quote about the 335 kW engines overboost feature from the Car Advice review of the RSPEC:-

"the Maimi produces about 375 kW for around 8 seconds at a time"

I know they can get good end speeds on a single 1/4 mile run under reasonable conditions, so I'd say the overboost can often last longer than 8 seconds.
Also with a period of reduced throttle operation the engine will recover for another burst, but I suspect it won't take too much hot lapping before going off the boil unless the weather is very cool.
So perhaps that goes a long way to explaining the result. The overboost is great for real world driving but not so good for hot lapping.

I've got to say though, the Commodore performs very impressively at the track considering it's power output.