I have an 2004 Gt, is there any advantages of removing both of the rear converters?

 

Only if it's done in conjunction with a catted 2-1/2" H or X-pipe, and a 2-1/2" catback--and then a premium fuel tune.

Just removing the rear cats (assuming they are not clogged up) is a total waste of time and energy from a performance improvement perspective...

 

Take em all off

 

^^^Agreed

 

If your gonna do it. Do it right and do it once. Buy a catted mid pipe or go with and off road pipe if you can.