The change in SFC from the JT8D to the CFM56 was a healthy 25% and more, the engine only weighted a little bit more and had better thrust/weight ratio.
The GTF isn’t so much better than the CFM56-5C.

Not to mention the GTF is still very much a developmental engine, whereas CFM 56’s routinely run more than 20,000 hours on the wing.

I confess I don’t understand all this talk about the GTF as if it were a real, up and running, in full scale production, engine.