No, Catback does SFA.. its replacing the pipe AFTER the restriction which is..uhhh the cat
MRT are FOS. You dont NEED a tune for any of the bolt on mods bar a turbo.
Sounds like scare tactics to me, id be skeptical of ANY difference from only a catback, other than some ear pleasure.
Of course though, to make the most of your mods you should look into getting one, mainly after intake + dump pipe, then id recommend a tune.
Even a tune on an otherwise stock car is a benefit, but you dont need to (or will you) make any more power because of the catback.
Besides, if you are after an ECUTek tune, go and see Tunehouse, they are forum sponsors and are trusted by the fussiest MPS owners here