The tiger is slow. but it's also one of the most durable tank and a powerful main gun.
the centaur by comparison have good anti-infantry capability but no where near the firepower or durability the tiger possess. This make a huge difference in their survivability as the tiger will easily scare away most enemy by its presence alone and have the toughness to survive getting shot at.
the centaur's durability is "okay" at best, but it lack the speed to ran away from most enemy.
Saying the speed is a safety to prevent over-extension is idiotic. Any smart or decent player can judge the situation and prevent himself from overextending. Only idiots would need a slow tank to prevent them from over extending.
indeed, the VI is a much better tank then the centaur, which is why it costs roughly twice as much.
for an AA/AI platform, the centaur is actually quite tough. it has medium tank hp, the best armour of the group, and very good damage. its durability is worse then a med tank overall but it also costs less then a med tank and has much worse AT capabilities.
the tank that i find the hardest to over extend, from being on both ends, is the V because it's tough, very fast (particularly with blitz), and can get doctrinal smoke. by comparison the VI B, which is exceedingly tough, i often see players over extend. i find that speed (pathing factors into this) is one of the biggest factors in over extension.
with all that said, what do you think the speed on the centaur should be, roughly?