The thing was slow to transform, was it's main weakness. It could have transformed into something more dangerous but
a) it was quite a cowardly thing
b)it still would have lost to flamethrowers, against which you can't really evolve a defence.
To try to be a hunting creature like that would have been more risky, and as soon as it splits, it becomes only concerned about its own survival. Otherwise the blood test would have failed and the Thing would have won.
Was it slow to transform, though? There was the guy on the dissection table, he could shoot up a pillar of flesh in no time flat. And then it turned into a giant monster in the span of a minute or two at best near the end. And that's not even the thing - it doesn't really have to transform in a visually impressive fashion. If the movie's anything to go by, the Thing is extremely infectious. It could just go around coughing its blood at people and it'd probably not even be detected, to provide an example off the top of my head. And, once again, it doesn't appear to obey conservation of mass.
Of course, you can excuse the Thing's poor planning by the fact it's not terribly intelligent and is generally animalistic despite its ability at mimicry of complex behavior hinting at considerable intelligence. However, hand the Thing to an inventive enough player, and they can easily curbstomp human opposition, given the advantage of surprise. Maybe even if they don't have the advantage of surprise, now that I think about it. That's what I was asking about originally, in fact - were the Thing's abilities downgraded from JCThing, or were they roughly the same?