It can be done. In my experience, 8 knots is the optimal speed/range ratio for a VIIC, although, of course, bad weather will slow you as you burn the same tons of fuel. But you have also a milk cow waiting for you, so keeping 8 Knots you should have a reasonable amount of time to create problems there
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AFAIK, one of the reasons for the construction of milk cows was to increase the range of the Type VII boats. In fact, Dönitz writes about VIIC uboats sent to the USA eastern coast after operation Drumbeat.
Good hunting.