There's a bit more to it than that. Like I said, clay courters get a boost because serves are much easier to return on clay. And on second serve returns, winning percentage is as much about rallying ability. Few would deny that Nalbandian is a superior second serve returner to Nadal, yet Nadal has a higher percentage of points won on second serve returns. This is obviously because of his ability to win rallies that start off as neutral. Nalbandian may win a lower percentage of points, but his actual return puts a lot more pressure on the server.
That 1st serve theory is very flawed: what if the server has around 30% of the 1 st serve in, in the particular match, how much of a factor would 1 serve return be if the 2nd serve return is bad?
We don't have valid data to measure the pressure that someone puts on a server as a returner (change in blood pressure, or growth of certain hormones or something else), without that it is subjective as it can be. I can speak for myself(depends on the surface): I would probably fear more that Nalbandian may rip me a return winner of second serve more than Nadal, but I would probably fear more that I will be broken by Nadal than Nalbandian if my 1st serve has higher percentage.
And Nalbandian isn't really a good example for comparing, cause his return stats are up there with the best, for example believe it or not on grass he has the best percentage of return games won out of active players.