Two inches of increased tire height will drop the Rpm by 200 at any given speed. One inch change in tire height is equal to .10 of gear ratio. Adding or removing an inch of tire height changes the rpm by 100.
I have 275/60/15's ("28) DR on my car now. I used to have 245/60's for the street which I think are "27 tall.
I have 26/10 slicks for the track and I think the car accelerates better with the DR. Maybe it's in my head?
I think the car looks better also. They fill out the wheelwell all the way.
I was going to ask the same question about changing to a 28" slick. opcorn:
I think the change is significant, especially true for drag radials. That extra height of the tire makes launching as different as night and day. As far as ride quality overall, the change will be noticeable, but minimal.