Well, you would have to be one hell of an uboot ace to win the war until '43.There is a limitation to how many ships per refill a sub can sink ...
I assume the outcome in certain battles can be influenced (like Malta) which might give you certain advantages, but the overall outcome is kind of hard to turn with the US and the SU pushing through.
Turning the war with subs would mean knocking out England (preferably before the US enters the war) and doing so would mean first cutting her of from her supplies. That only works when the numbers of uboots is right and the commanders are real aces. Since you are the only one that "changes history" here and certain aspects like sonar, radar and the enigma problem remain, the possible impact on the overall supplies is rather limited. *think*