I believe the "underpowered" nature of the .30 Carbine is, like a lot of commentary in the gun world, pure rumor, and I'll think that until I see extensive reports saying otherwise. On paper, of course it's weaker that the full powered rounds, but by no means is it weak, from a ballistic point of view. And, again, it accomplished what the military needed it to, so no complaints. Stacking it against actual battle rifles is unfair though.
There's also this for what it's worth:
http://www.youtube.com/watch?v=a7ktaTbgQsU
Not sure at what range that is, but that's 6 layers of wet denim, and a 6 inch wooden block, and it goes clean through, so anything on penetration I'll about dismiss. If you say it's underpowered too, you have to say that the .45, which is ballistically weaker, is also underpowered, and the ever potent 7.62 Tokarev, is underpowered too.