No, knowing about Rivet City saves you absolutely no time whatsoever. It is exactly the same as if you pass Threedogs speech check and get him to give you the information for free, it's just another goddamn waypoint that gives you nothing of meaning, unless finding out that Madison Li is a complete cunt is somehow a positive for someone.
Even if the exchange isn't completely equal it should still be at the very least acceptable, and the quest structure of this example in particular is egregiously bad, because the resources you expend simply allow you the opportunity to expend more resources for another opportunity, that, and here's a shocker, simply allows you the opportunity to expend more resources for what at last becomes the information you actually need. And at no point in this chain does the player receive ANYTHING to actually help them, Threedog says thanks and scoots you out the door (right back into a super-mutant and ghoul infested urban ruin), then Madison just sends you to another super-mutant infested ruin with total disregard for you as a human being, and finally, after three arduous treks through the wreckage of DC and brawling your way through the purifier, you finally get the information you want, which just sends you to the opposite side of the worldspace, to enter a simulation run by a twisted psychopath where you have to jump through more hoops (or massively sequence break) to get James and yourself out.
The main quest of Fallout 3 is completely comprised of 'fuck you, go do this now'. And to my perspective that is absolutely terrible design philosophy.
And no Rolan, the BoS do not help in any meaningful way whatsoever when you go to the museum of technology, they might gun down a mutant or two outside but you are the one who has to go inside the building alone, kill all the mutants inside, get the dish, and install it. This is enormously expensive in terms of the resources you have to expend, either in the form of weapons and ammo (and stimpacks) or stealthboys. And the payout is, "Go talk to Madison, later" and booted out the door into the DC ruins.