The answer is.....nothing visually that we could detect, but two additional steps on a ladder.
Last dark window our club (Chesmont Astronomy Club) went to Spruce Knob, WV for several nights of 'black sky' viewing. East of the Mississippi it doesn't get any darker than this, folks !
I set up my (Webster) 24" next to my buddy Josh's 25" Obsession ( mine is a Kennedy f/3.6, his is an OMI f/4.5). During the course of the evening, we compared views on Arp 166, the Dumb-bell Galaxy pair (NGC 750 and 751) using his Pentax 5mm. We used the same EP to eliminate it as a variable in our comparison Arp 166 is a fairly challenging object, and a beautiful interacting pair of galaxies in Triangulum. So I was at 450x and Josh at 570.
Josh is a very astute observer, as are many others in our club. At the end of the day (night), no one could discern any optical difference in the views of the two scopes. The extra 1" of longer focal length did not appear to provide any more detail in the galaxies, or in the surrounding field stars. This is one view on one object (albeit challenging) one one night with average transparency.
The major difference is that I'm two steps lower at zenith than Josh, and that is HUGE! So much for faster mirrors......
Sorry, next time I promise photos - but I arrived late and left at sunrise, and didn't get pics of the behemoths.....That night we had a 25", 24", two 22" and an 18" dob....Firepower on the mountaintop!!