I believe that keeping ESP on in the snow is better solution. It keeps you a bit on safe side for unexpected. In the contrary, trying to have some fun in the snow, in controllable conditions (track, I want to drift) swithing it off might make perfect sense. Also bear in mind that switching ESP off is not really switching it off, but reprogramming it to off-road mode, so it is still active, but working differently. I'm not sure how it is in MY'13, but in '11 off-road mode does not equal to driving in snow...
On winter tires, well, there are good summer tires and bad wither ones... Personally I'd never drive in winter on summer tires. For many years I was driving in heavy winter conditions and I know the difference. Obviously in most cases you can drive in the same conditions on summer ones, but you will know the difference. And in some cases winter tires can save your live, where summer ones would not do. Your choice.
P.S. I found this forum few years ago at the time I started to consider to buy Q5. I'm visiting it almost daily and usually I'm able to find tons of useful information and advices here. But recently, frankly speaking, I become embarrassed to read some post in this, once reputable and personally I think THE BEST Audi Q5 dedicated, forum. I understand that people might disagree and sometimes, due to language barrier (also for me English is not primary language) might understand some post to be offensive, directed against them. But this forum is to share our Q5 passion, not personal animosities. I am, and I'm sure that most of other members share my opinion, not interested in such discussion! This is not why we come here. If you really need to fight, please make it off-line, via PMs. Please, please do not destroy this community!