I've been here over 5 years and I've never heard of anyone having his(her) teeth smashed in for speeding. Far from it, driving tends to be a "free for all "experience here and nobody seems to get stopped for overspeeding.
The police are tough with drink-driving though. If they stop you for driving dangerously and you fail a breath-test, it's the slammer for you. Frankly, i agree with that 100%.
Taxis are fairly cheap so there's no excuse for not getting a cab home after a wild night.