How common is it for patients with serologic evidence of delayed hemolytic transfusion reactions to experience clinical symptoms?
So-called "delayed serologic transfusion reactions" are much more common than delayed hemolytic transfusion reactions. In the latter the patient has biochemical evidence of hemolysis, e.g., anemia and hyperbilirubinemia. See, for example, this study by Heddle et al.
Note: Heddle's study, reports 95% confidence intervals (CI) for study results. For example, of the patients who had detectable alloantibodies pretransfusion, "8.9% (95% CI 3.6-17.4%)" developed additional antibodies. This means that based on the study's sample size and the results, there is a 95% probability that the true percentage of patients who produce additional antibodies is somewhere between 3.6% and 17.4%. When reading literature it is important to take note of the 95% CI.