I had the inertia reel type fitted on my F172H a couple of years ago. The fitting was carried out by Mann Aviation at Fairoaks during its annual. In fact as I recall it was recommended by the engineer dealing with the aircraft.
It looked like a very sensible safety precaution to me even for a tall pilot. I’m around 6ft 3.5 inches. Seats unexpectedly sliding about at a critical phase in flight may well risk dangerous mishandling of the aircraft even if the pilot can reach the controls.
I strongly advise shopping around on price. I don’t believe I paid anything like the cost you have been quoted.