Yes, there is a difference in performance.
Is better use a SUM function of 1, than count. So, as you explain, is better to do SUM(ClientLeftFlag) than COUNT(ClientLeft), asuming that ClientLeftFlag is 1 when the client is no client anymore.
Sometimes we can't apreciate the difference in performance, due to an application with few rows. But when you work with really big applications, you would notice that difference. Also is a good practice to do it, unless you know the performance won't be affected.