You mean the attempt IOTL?
https://en.wikipedia.org/wiki/Operation_Pastorius
http://listverse.com/2013/06/12/10-nazi-spies-and-their-espionage-plots-in-america/
Or the earlier spy ring that was broken up?
https://en.wikipedia.org/wiki/Duquesne_Spy_Ring
If you want Pastorius to work you'd need German intelligence to do a much better job in terms of selecting loyal agents. It would also help if the Duquesne ring isn't broken up too. Really the only reason the Germans failed in their spy missions was bad agent selection; the US counter intelligence efforts were non-existent and the Germans, had they actually been better at their craft, could have done DAMAGE with their plots.
Even if they just did a handful of their missions, like the Niagra Falls plant and a couple of the Aluminum plants they would have seriously damaged the US war economy as it was just gearing up.
So if we assume that things go according to plan due to better agent selection the Duquesne spy ring is still in place come 1942, not having been betrayed by Siebold, and the Pastorius agents come in, again not being betrayed by several of their own guys within the mission. Given the total lack of FBI awareness of German efforts without those betrayals and their pretty much non-existent counter intelligence capabilities the German agents could have run wild doing major damage for quite a long time. There was no NSA, the OSS was just being formed, and the FBI had only a few thousand agents nationally that were mostly focused on fighting Communists.
I'd say that they could have easily postponed the Torch landings due to US fears that German agents have found out about it, as they were enormously cautious in their efforts IOTL. The level of damage at home could be quite extensive actually if the plans for Pastorius actually got off the ground. The US would be in a massive panic and I really could see the FBI turning to the Mafia to help them, especially the Jewish mob. IOTL the Jewish Mafia pretty much destroyed the German-American Bund as an organization:
http://www.thedailybeast.com/articl...es-mob-s-fight-against-nazis-during-wwii.html
http://spectator.org/articles/37005/operation-underworld
Frankly the Nazi agents would wish they were captured and swiftly executed by the US government first in that case given some of the stories I've read about Mafia 'justice'.
But in the meantime if they did start their mission, they'd have months free to wreak havoc and paralyze a lot of US decision making and war preparations until they are run to ground.
actually no, the Brits had no idea. The mission failed because several agents promptly turned themselves in. It actually took them a while to convince the FBI that they were German agents in fact, as the FBI really didn't care, as they were focused on fighting Communists rather than Nazis.