I think KEF Qs are a step up above the Polk RTiAs. Though I never listened to the Center, the Q950s were impressive, and I know the 750 met with some strong accolades.
Regardless, I always tell people: "it's your system." You have to make the decision that's right for you in your home.
Though it might burn a weekend of futzing around with it, I suspect you'll be surprised if you can get the center onto the top shelf and below the screen.
I do have my screen mounted above an upright center. Bottom of the screen is ~44" from the floor. My ears when I'm sitting up are 49". I do not find the height of the screen problematic or uncomfortable.
I only share this as encouragement to experiment. Considering the cost of these TV legs, I would try if our roles were switched.
If you try, and I hope you will consider, get the center foraward on the shelf, about 1" overhang for the front baffle and a slight upward tilt if needed to fire at your shoulders. Try to keep the screen about 1' back from the front of the Speaker, and just high enough so you aren't blocking it or the remote sensor (my setup right know does block the sensor a bit, but I'm a music first guy and don't really care if I have to raise my arm to shoulder level to control the TV.
).
Again, just friendly encouragement. May you have a successful adventure if you do!