I was down at a knowledgable rolling road tuner / rally car place yesterday.
We were sorting the carbs on another car.
The owner was interested in my Astra. So we got talking about tuning.
He reckoned it'd be worth fitting a manual distributer to my engine so he could adjust the ignition timing. This appealed to my scrap yard tuning instincts.
It's the 1.6 Cavalier units that fit isn't it? I'd take the distributer, Hall sensor and wire it in. The ECU based ignition would be ignored.
The other interesting thing was that he said that in some cases a rising rate regulator (like the FSE power boast valve) does yield quite good results. Obviously not 10BHP, but a few BHP at the top end. Depending on it being properly adjusted.