Totally agree. Would be a waste of money when you take into consideration the parts cost, and of course the labour.By the time you've added this on to the A5 it would cost more or less the same as an S5. You would be better off getting the S5 and it would have far more grunt. Trust me I used to have an R32 and the S5 is something else.
Maybe on Used cars it's an idea, becuase there might be a decent gap between the car values in a few years time (3.2 vs S5). But brand (or nearly) new, I don't think there's any way to justify the costs of it.
Plus, you can kiss your engine warranty goodbye.
Why not consider an A5 3.0TDI, and have it remapped? You could be looking at a 0-62mph time of 5.1-5.2secs, power output of around 290BHP, and a good increase in torque.