More thoughts:
A lot of what the big head alien explains is the same as the god-child explains at the end, so if you have the DLC, it seems a bit redundant then. Learning that information earlier should affect Shepard's talk with the god-child, but I'm sure it won't.
The big head alien confirms that Harbinger was the first Reaper and emphasizes it. The "you lose" 4th ending implies Harbinger is the god-child and this kind of confirms it since the god-child purposely sets itself as something apart from the Reapers, i.e. the intelligence that came before them, the one programmed by the big head aliens.
Makes me wonder if they're gearing up for some kind of Harbinger-focused DLC. The big head alien says the Reapers are looking for something, which is probably synthesis, but the current culture in Bioware's Mass Effect writing team seems intent on having to explain everything so we'll find out. But I don't think they'd underscore Harbinger's importance for no reason when the ending's already been done.
Side-note: I'm guessing Bioware put in the line about each Reaper being made in Harbinger's image to shut everyone up about how all the Reapers look the same despite EDI's theory about shape in the Reaper reproductive cycle... but then why does the one in the Collector base look human? Still stupid. Also, if that's the case, why do the Harvesters look like griffins? There's no reason to put useless, flapping mechanical wings on those things. The flapping accomplishes nothing. Stupid again.