The RN was mostly focused around the North Sea, in order to contain the German HSF. If the United States joined, it is doubtful that the British would not be aware of such political shifts and developments, and would have prepared for naval combat to stop such an action. Besides, most of the American fleet exists in the Pacific. The ability to transport any troops would depend entirely on the material of the American navy at the time. Canada also is an issue. Had the US been preparing for such a conflict? Had Canadian reserves been left behind on the New England border/Toronto Corridor for such an event? Would Mexico take this chance to work with the US or would the divided attention of Washington mean that the Mexican situation would become unpalatable, leaving the US with enemies on every side?
The United States joining with the Central Powers in WWI leaves the US in an undoubtedly weaker position in every sense. It is more likely the US would have threatened Britain with intervention if it did not allow American goods to travel to German ports. This probably would not have the desired effect, but the chance exists it might. Only then could American entry into the war seem entirely feasible.