You are in fact correct with a Disel being one of the hardest motors to make high performance but if you have noticed in the past 5 years Opel is putting high performance disel's in the market. The Speedster's orginal concept motor was a turbo charged Disel making somewhere around 400hp (don't quote me and if you know the correct spec let me know). The other factor is the Opel is twin turbo charged adding emmense boost pressure air input while the Enzo is normal aspirated and has to have air normally feed into the chambers rather than having a booster (turbo, supercharger) to force the air in creating immense power with out taking up a lot of space or having to run high RPM.
Bear