The difference between an IQ of 100 to 130, is not the same as the difference between an IQ of 70 and 100. Yes, the gap is still 30, but i don't think the test works that way, the OP's example doesn't really work that way in the real world.
Someone who has an IQ of 140 is not "twice as smart" as someone who has an IQ of 70, the curve is exponential and gets steeper the higher you get.
It's much harder to go from an IQ of 140 - 150 than say, an IQ of 60 - 70, and gets harder exponentially the higher up the IQ level you get. However in saying that, people of low IQ will likely have less ability and determination to reach a higher IQ level, where as people with higher IQ's will be more likely to strive to acheive an even higher IQ.
I'm not sure where I'm going with this, but it sounds good to me, and it's used up my whole years quota of saying the word "IQ" in one post.