Steel is overweight in this case.
"Wide and low profile"? This is the mid-nineties we're talking about. Piech had a bee in his bonnet and was supported by his engineers. Vorsprung durch Technik - still meant something at this point. Please: read the book. The 17s were a *special option from Quattro GmbH* and only 7J.
The aim of the game was the first five seater four door 3l car. There were tax incentives in place at the time. If you don't know the history, drop the subject.
And no, you couldn't change tyre sizes without losing the 5l status, which was worth something - and 3l status essentially meant tax-free, which was around €400 / year at the time if I remember correctly. This is why there's a big discussion elsewhere talking about 3l, D4, and all the rest of it, as it was worth quite a lot of money at that time.
The reason the tyre / wheel combo starts with 15" is to deal with the requirement for reasonably large discs for braking performance. Anything less than 7m2^2 deceleration was unacceptable at the time in automotorsport, and it was *the* magazine. ams and Bild basically dictated what would sell and what would not. ams would test 10 times from 100 to zero, one after another, and 7 was their limit. Autobahn, high speeds, decent brakes are essential and anything that wasn't up to scratch would be panned. Note that the 155/65R15 in the original CoCs is only for M&S tyres to allow sufficient space for chains. Many ran these in the first years; 175/60R15 was rare, 165/65R15 essentially didn't exist but the paperwork said what it did. I remember cursing the cost of the Semperit 175/60s I bought as my first set of winters (on steel wheels).
The reason the ride is so stiff is because the A-Class Älgtest had recently happened, so a bunch of changes were made and every single A2 had ESP fitted from the factory (and tested!) before being let out. This is also the period with the retrofit of the TT with ESP and its spoiler, which was unheard of until this point. It was complex and costly but seen as the only way out. Essentially zero A2s were sold in Germany with steel wheels from the factory; it was possible to do but they're really rare IMO.
The weight differences between tyres will be minor but significant, depending on exactly what you're trying to do. If you are looking for balance, then binning the rear seats is the wrong way to go, as the TDI is already nose-heavy. There's a reason there are weights behind the bumper in some cars. But assuming a single size weighs something is also silly as there will be differences between makes and it's not immediately clear just how much.
