That is your imagination. Check out Ashwin's stats before he went outside Asia and how it got battered there. Let us compare Ashwin and Yasir on common wickets they have already played.
Ashwin averages 19 in Bangladesh. Yasir averages 34 in Bangladesh.
Ashwin averages 18 in SL. Yasir averages 19 in SL.
The sample sizes are small, but that is all we have for strict comparison. If Yasir is better than Ashwin then why he didn't do better in Bangladesh and SL? So all this talk about Yasir being better on unhelpful wickets is speculation. If my intuition is right, Yasir will average 40-50 during the Australian tour and will have his stats ruined to some extent - this is what happened to Ashwin during his early tours outside Asia.
And we have not yet begun to talk about Ashwin the ODI bowler and T20 bowler. And Ashwin the test batsman averaging 32 with test fifties outside Asia. Ashwin the player is far ahead of Yasir the player across all formats at this point of time.
You mean Ashwin's first 12 matches? He was nowhere near Yasir Shah is at this stage.
Performances against Bangladesh are removed when comparing players, I think this was an established procedure. Sangakarra is dismissed as a minnow-basher but in Ashwin's case, Bangladesh is the standard for judging how good a player is? I don't think so.
In Sri Lanka, you can't pick one over the other. They were virtually equal, with Ashwin having slightly better stats but also a better supporting bowler. These were a total of five or six matches, with both bowlers in top form in Sri Lanka. Just because Ashwin was as good as Shah in Sri Lanka, doesn't mean that he'll keep this up for the future.
As far as neutral venues go, they have been equals. When you consider that Ashwin bowls on much easier pitches at home than Yasir Shah, you have to rate the bowler who is making things happen due to sheer skill, not because the pitch is taking the wickets for them.
Yasir may very well average 40 in Australia (which will still be much better than Ashwin's performance) but he's a leg-spinner and by definition, will get more turn on unhelpful tracks than an off-spinner will. Shane Warne has said that Shah has the best leg-spinner that he's ever seen. He's already getting the job done on the flat tracks in the UAE, with a little extra bounce he shouldn't be out of his depth, as Ashwin was. We shall see what happens.
It isn't like other spinners haven't been able to better Ashwin's performances out side the subcon; Ajmal was much better in England and South Africa and so was Swann. Yasir may or may not turn out to be as good as those two but it isn't impossible for a spinner to perform well in difficult conditions.
I have already said that Ashwin is the better LOI bowler and if I were to pick someone for my team, I would probably pick Ashwin due to his batting, although is seems like he's no better than Yasir, with the bat these days. Unlike some people here, I am not merely arguing for Shah because he is Pakistani or because I have a picture of him as my avatar but because I genuinely believe that he is the best test spinner on the planet, right now.