Congrats and welcome!
The Convertible does have extra bracing underneath to prevent the cowl shake, but as your car is 12 years old, it may be best to get underneath and give all bushings a good check over as you might have some tired bushes. Not heard particularly of cowl shake issues in the A5 cab.
I did make a small batch of firewall brace bars that sit under the plastic scuttle cover (where the brake reservoir, wipers etc is). This did taught up the front end a bit and this was on a coupe. I can't remember exactly but there was a US based guy making these bars called Canyon Run CR-15.
There are upgrades for the cross brace under the engine too. So plenty of options for the A5/S5!
Maintenance on these is pretty straight forward. General servicing, some say to reduce the oil service mileage, especially if tuned. Gearbox service (presuming yours is auto (DSG)? Box service every 35k miles - the early gen boxes (Pre 2012) were renowned for issues, but mainly sensors and mechatronic pressure causing gearbox errors - but the parts were updated and most cars by now have been upgraded.
Performance upgrades - plenty to chose from and plenty of tuning companies from stage 1 Remap only giving around 440 bhp to stage 3 giving well over 500bhp - depends on your wallet! Most popular are just a remap or stage 2 with intake mods and pulley upgrades for the supercharger giving around 460-480bhp that leave the wallet happy!
