What do you think of the non-leather sport seats?

I can only comment on the standard part leather s line seats in A3 I test drove last week, and I found the roughest roads round my way to test the suspension and found that the seats where brilliant at helping absorbing the bumps without bouncing you around. Far better than my Honda which I thought way pretty good until I compared it to the s line.
A3 s line standard (same as sport but upgrade cloth / leather i believe) are really comfy but not too spongy / springy.
Have only sat in a stationary BMW 1 series and found it a bit cramped and not as comfy as A3, but i am 6'3" and quite fussy about seats and seating position.
not quite what you asked but hope it helps a little.

Yes, I have Sports Seats. I am a heavy individual and they support me very well. I have only done a two hour trip so far in my new car and I have found them just fine.