I just bought a '04 147 GTA and have already had a large 'scrape', it disrupted the function of a reverse light  .  I will definitely be investing in an aluminum under tray and will not be lowering it.

I am very lucky, the previous owner has a lot of experience setting up 147 race cars.  He didn't change the suspension and had no plans to.  Instead he bought an uprated strut brace, which eliminated all body roll, the handling is stunning.

Other performance mods are:
Q2 diff.
Hi Tech racing exhaust, fantastic sound if a little loud.
330mm Brembo (front) w/ racing pads

PS. I'm not sure if the previous owner wants to be named. Many thanks to him anyway.