Both CEO and NEO A320 variants will be thrust limited (as opposed to buffet limited) at the rec max altitude. However the NEO has slightly more thrust and generally isn’t derated for airway charge benefits (in my company anyway) and therefore at it’s rec max limit it will be closer to the 1.3g buffet. I Imagine this situation or something similar with regards to optimum fuel flows for the different engines, plays out in some form at the optimum level as well. If you’re flying higher or closer to the buffet limit then the margins will be less.
I haven’t noticed the sluggishness that you describe on the NEO.