15" or 16" were the 2 rim sizes that Audi designed the A2 around, the 17" wheel is actually an afterthought and sourced from the old-style A3 (it is an 8L part number).
The harsh ride is caused by a combination of suspension and wheels and swapping out one or the other will improve the solidity somewhat.
I used to have an SE spec A2, and the ride was comfy, but firm, or so I thought before I got my current Sport model!
The Sport is a firm ride, which makes the SE feel soggy. After a big discussion with Audi UK over my 4 buckled 17" wheels, I have now got TT 17" wheels, with slightly taller tyres, which improved the comfort considerably, but then I went and fitted a set of H&R progressive springs, which now make the originals soggy!!
So, it depends what you want, in terms of ride quality:
If you want ultimate comfort, go for 15" wheels on standard suspension. A bit firmer? - opt for the SE spec. Firmer still? The Sport pack is the way to go, but for ultimate go-kart handling, choose aftermarket Sport lowering springs and try to fit as tall a tyre as possible.
Personally, although most people (my significant other especially!!) seem to think that Tank is now too harsh, I have to disagree - on a well maintained road, such as we have the right to demand, in view of the amount of transport-related taxes we pay, the lowered, stiffened suspension and wider 17" wheels are an absolute dream and are as comfortable as you could wish for.
It's only when you get onto a poorer surface that things start to suffer - like my back!!
Rant over,
Cheers,
Mike