Yes, monitoring for Marfan Syndrome probably makes Michael Phelps a better swimmer. He obviously has a family history that makes him concerned about the disease, so monitoring for that would be essential to his health and training. Marfan causes cognitive problems as well as difficulties making muscles work. If Phelps were to develop the disease, it would be detrimental to his athletics and require help.
It is understandable that Michael Phelps would visit Johns Hopkins on an annual basis to check for signs of Marfan Syndrome. His long slender build is common among those who have the incurable disease. Monitoring his health is a good idea, but being healthy does not make him a better swimmer.
Montoring his health for signs of Marfan Sydrome is not about making him a better swimmer, it is about taking care of his health, so that he does not die at a young age from what is a very serious illness that often kills people who are in thier 20's and 30's.
No, being checked every year for Marfan Syndrome does not make Michael Phelps a better swimmer. It does not make sense to believe that being monitored for a disease makes Michael Phelps a better swimmer if the disease does not have anything to do with swimming. This is good for his health though.