Don't know if anyone was interested, checked my figures and I had them slightly wrong Here's the text:
The 2005 Opel OPC Vectra GTS developed by the company's internal tuning group, Opel Performance Center (OPC), is expected to feature GM's first two-stage (or "register loaded") twin-turbo common rail diesel in a light duty vehicle. The result is a remarkable level of power and torque from a modest displacement. The new 1.9L inline 4-cylinder generates 212 hp (112 hp/L from a diesel!), and approximately 400 Nm (295 lb-ft) of torque from 1400-3600 rpm. The two-stage twin-turbo setup allows each turbo (one small, one larger) to contribute boost across the rpm range for optimum pressure charging. The low inertia of the small turbo permits rapid boosting below 1800 rpm, while the larger turbo contributes a high magnitude of boost above 3000 rpm. The rpm range between 1800-3000 is the transition range where both turbos overlap. The Vectra is based on GM's global Epsilon FWD platform, shared with the Chevrolet Malibu/Maxx, Pontiac G6, and Saab 9-3. GM claims that the improved output from "register loading" does not result in higher fuel consumption compared to a single-turbo modern diesel of the same displacement.

I expect expense is the only reason it's not common on petrol engines. It's remarkable how the diesel is being transformed from a tractor to performance engine.....