Sleeving individual cables from a 6/8/24 pin molex plug will cause more airflow turbulence, since the overall diameter of all the cables added up is greater than sleeving over the entire bunch.

Something I found recently was adhesive backed cloth tape. Wrap this around your cables where they fan out from the sleeving, then put the heatshrink tubing over the transition. It creates a nice clean look with minimal disturbance of the airflow.

I found the cloth tape (black) in the sports section of Wal-Mart. I think it's used for wrapping sports equipment handles.

Steve