Put simply, the craziest driver will probably win and I wouldn't recommend racing on public roads.
However, I've owned both, in fact my Scooby was a WRX Prodrive so had 235 bhp, i.e. less than an STi. On twisty and bumpy british roads, which let's be honest is most british roads the Scooby would win hands down every time. The 4 wheel drive and suspension setup is much more suited and even my WRX was quicker. Also don't forget the Scooby is about 250 kg lighter so the power to weight ratio of an STi is better than a Zed, so even on the track I would also say the STi would be slightly quicker.