For raw speed and grip obviously the Audi is on top. I came from an S5 which i had for the last 5 years but due to a child i needed 5 doors (excuse to the wife ha) and its a fair power hike on the s5 which was very equal to the supra i'd say.
The RS range have all the buttons to play with regarding suspension and engine modes etc so while everything is in comfort you can tell it's a big car but when you put it into dynamic it all hardens up and becomes a bit of a monster, at this point it compares to the supra a little more imo, everything is noisier and harder on the road and the steering is a lot sharper.
The RS has the dual clutch triptronic too, gear shifts are instant it feels very nice at high speed, id love a gear change that fast in the supra.
As you say they are worlds apart and difficult to compare, my supra is only 1bar stock engine'd pretty much so not much comparison in the speed department but they are both pretty nice cruisers.
But you can't beat the turbo noise of the supra
Cheers guys.