those astra VXR's aren't slow, easiest and crudist method would be to work out the power to weight ratios on the car.
350z has minimum 275 bhp, but weighs in at just over 1600 kg. so its bhp per ton is only about 172 bhp per ton.
something like a (going on previous car) mr2 roadster has 138 bhp but only weighs about 900kg so has 154 bhp per ton only about 18 bhp between them when you take the weight into account.
i had friends who had turbo'd theirs and were running at about 280 bhp or more accuratly 311 bhp per ton. and would trounce an untuned zed.
our only saving grace is our huge torque, most little engines have to work the gears and some rev higher, but we can pull alot harder in each gear.
but at the end of the day, if the driver of the better car is crap, and the other driver is better you may not do very well on the old back roads.
hope that makes sense.