But you almost always ARE trashing your Rats? I don't think this hypothetical is very useful; the real question 90% of the time is how well your Rats are keeping up with the trasher. At most you might delay buying your TfB card but even this is hard: often this means putting your deck on hold, and also it's hard to buy Upgrade with a hand of all Rats.
The problem in the case where you ARE trashing is, it depends a lot on shuffle luck whether your one Rats turns into 2 or 0.
Also, the fact that the Rats grow exponentially is a strong argument for the efficacy of tossing an extra one in at the beginning. It's very effective if you want to speed things up even further.