There is a lot of difference in ride quality depending on manufacturer and load index. Higher load index = stiffer side-wall = harder ride.
I just changed from Avon ZZ3 to Michelin Primacy Alpin on A6 both with the same load index of 93 ( minimum is 91 for that car) and it feels like different suspension- a lot softer on bumps and potholes but the same or better stability in the corners.
As for the speedo -on some cars you can calibrate it, have not looked it up for A8 and all/most cars show higher speed on speedo then actual with standard tyres, check yourself comparing to SatNav.
Also you need to inform insurance company if you changing to non standard tyre size and it may attract additional premium.
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the car is tracking all over the place as in it feels like a real effort at times to steer the car because it seems to want to follow all the ruts in the road and chase the white lines
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You might want to check suspension bushes and wheel alignment for this- full alignment not just tracking.