I've had mine since December and I still can't believe the huge torque you get at low revs - very handy when you're in the wrong gear.
So far, I've found general cruising gets me 30-40 mpg but, due to the huge mpg drop when accelerating, most journeys average in the early twenties. I have noticed that staying in too high a gear and accelerating from about 1000 rpm gives better mpg than dropping down a gear. Not sure how good that is for the engine/gearbox but the torque's so great it's really usable at those low revs.
It's a pity you've had all these problems but, if all's okay now, it won't be long before it's a distant memory and you'll only be thinking about the driving pleasure you'll get when you see the S4.
I've never driven an RS4 but I'm not sure they can't be that much better on the road - the acceleration of the S4 is almost too much (verging on excessive in some cases) and, being an Impreza owner, I know that you'll never get anywhere near the handling capabilities of good performance car on the road as it's just too dangerous to be going round blind b-road corners at those types of speeds. I like the S4 for the acceleration and refined ride.