Think positive. How many times did you sink an entire task force in SH3? Or an entire convoy?
I imagine that the action fan playing with 0% realism options will be able to sink a LOT of ships and might probably win the war. But if you play with 100% realism I doubt you can achieve such a thing and so you won't win the war, assuming the devs carefully balanced the system.
I'm with Sober on that one. I think that if you play with realistic settings, you will have a realistic influence on the war, which is close to nil except local effects like shipping or escorts redirection. But I may be wrong too, damn, March is so close yet so far