That logic doesn't hold up here .. even if you slow down TN further, passengers will always prefer a train that reaches early at NDLS than a train that makes them half a working day. They will have to slow down TN by a lot to make people prefer Raj.
Besides, Raj is just a bi-weekly. Any change in schedule of TN without changing the pattern (2 nights, 1 day) will not impact Raj in anyway. TN is already more preferred because of the pattern.