Craig - the 20XE/C20XE engine fitted in the Cavalier GSi was a classic. 150PS in standard form and easily tuneable to acheive much higher. The engine was designed, and partly built, by Cosworth and originally intended as an homologation special only which is why it was electronically limited.
The C20XE sold so well that GM Opel/Vauxhall commissioned Cosworth to productionise the design to become the 136PS 16v Ecotec X20XEV engine that was fitted in large numbers in lesser Cavalier, Vectra and Astra models such as Adrians.
The original 20XE is still preferred by Vauxhall modifiers but these engines are all getting long in the tooth now. Many have been converted with a turbo (and other mods!) to give up to 350bhp, which is a tribute to the strength of the original 20XE.