Well, if 1% of Christians in general become Quiverfull, and 8/5=1.6 Quiverfull children per mother remain Quiverfull, you get 1% first generation converts, 0.99 x 0.01 + 0.01 x 0.8 = 1.79% second generation ...

Of course its more complex than that, and many of the children who do not remain Quiverfull will still be 'carriers'. But without working out the equations, it still seems clear that genes that predispose people towards Quiverfull will have higher fitness, but its not going to quadruple every generation.

Essentially, the quiverfull people aren't spreading memes well, but are spreading genes that predispose religiosity.

And sure, your evangelical friends might not think it would work, but then they probably don't believe in evolution.