While the design may have been around for 50 years, they are still modern engines. GM is capable of producing multi-cam V8s, it has produced them in the past for racing purposes. While the pushrod design is inherently less efficient than an ohc/4 valve design, they have advantages in other areas. A pushrod V8 is much smaller allowing it to fit into compact engine bays, it is lower than an ohc design lowering the centre of gravity and when made from an alloy like the chevrolet design is relatively light weight. They are also cheaper to produce.
As for the M5, i grant that it has enough power to do what it is designed to do. But from an ownership standpoint i would be a little upset that a LS1 engined commodore with very basic modifications would trump my $200K BMW in a straight line.
Don't forget when comparing commodore times to German times that the holden's crap rear suspension hampers it's launch significantly. Comparing roll-on times would give a better comparison of the engine strength of each car. Also remember that the Germans tend to give conservative power estimates with their cars. Eg, the SL500 has probably 50-70kw more than its quoted 220kw.