Currently The extra power of the X doesn't matter and next gen, any differences won't matter either. Purely because devs just haven't got the time/ money/ urge to optimise their multi plat games for each and individual platform.Its 43% difference in case of xox and ps4pro (also bandwith is 1.5x on xox), so yes in this case it matters. 14tf to 12.5 is 12%, almost impossible to notice.
Yes, there might be a slight difference in 1-2 FPS, sharper this, smoother that, but unless you look closely, you will probably not notice.
Some devs do make the effort and games like RDR2 were a good indication but that was very rare so far. Rage 2 is a big letdown. Not only is the IQ very bad, the extra power of the X is nowhere to be seen.
First party exclusives will be a different story and will take advantage of the full power of next gen consoles.
MS putting in 1-2 more TF in their Future console, just to have the title of the most powerful console again, will most likely not matter at all if the current gen is an indication.
And as you said, we are talking over 40% where next gen we are looking at around 10-15% if at all.