The whole rear suspension was checked before the dealership let me take the car as I had to replace the whole rear subframe on my last GTV and that was fecking expensive.
Contact only happens at speed I think this is probably why people upgrade.
Also got a new clutch and disks as they didn't want to sell me a duf car so thedy seem pretty thorough.
Bottoming out the suspension at 60/65 mph don't sound right especially if it is on standard Alfa set up. I can't beleive that when they took a Cup with 17" teledials round a track Alfa would have missed that! (mind you they may have just put it down to the Alfa experience!).
Not sure what it is but it ain't right! I'd suggest taking it to a half decent garage/back to the dealer for an inspection.
Bushes going are a common problem (upgrading to powerflex being a common solution) but I'm not sure if that would cause the bottoming out you described, more of a clonking noise gives them away.
Are you SURE the noise you're hearing is the tyres hitting the arches? Not just an undertray hitting the ground? Although the ride on the Cup is lower, I'd expect it to be firm enough to stop it bottoming out, assuming everything's as it should be.