I found this page:
http://www.acc.eurobell.co.uk/mycar.htm
Where (as well as details of his previous Citroen ZX Volcane) he gives the differences between the A5 and A8 ("after much research") as:
# Air intake ducting/filter housing,
# Camshafts with increased lift,
# Catalytic converter is a larger size,
# Different pistons giving a higher compression ratio,
# Enlarged throttle body from 60 to 62 mm,
# Exhaust manifold / down pipe,
# Exhaust piping has an increased bore of 57 mm instead 51 mm,
# Flexible pipe before the catalytic converter is shorter,
# Optimised ignition and injection timing,
# Valve seats readjusted from 60 to 45 degrees,
# Last but not least, the valve cover is red.