I've just recently gone to catless downpipes on my b5 s4 that I have tuned to stage 1 currently.  I have noticed a good amount of grey smoke and smell since doing so especially when coming to and idle after driving hard.  My clothes also smell like a weed wacker engine.  I'm also hereing backfiring when revmatching and such but the pipes are non-resonated and catlesss.  My thoughts are it is running rich which is producing the excessive smoke?  Slow oxygen sensors or is a leaner tune needed to compensate?

I had a very similar perception when I removed my cat (1.8t). It still stinks but the AFR reads fairly normal at idle - when it smells.

http://nefariousmotorsports.com/forum/index.php/topic,1160.0.html

Definitely sounds like you're running too rich...  What does your WBO2 say?

Check your plugs for oil fouling.  These cars run reasonably well even with fouled plugs.

I suspect valvestem seals.  Mine cleared up after an italian tune up - I suspect the seals had dried up having been parked for 5+ months in a garage where the temp was over 100 degrees around the clock.

If the turbo seals were leaking it would smoke all the time.

That's classic valvestem seals.

In my case my valvestem seals were leaking enough to foul the plug and lay down pretty impressive fog.  It was not something I even knew was going on until I pulled the cats.  I've been running 15w50 Mobil 1 for the last 120,000 miles or so, before that it was 0w40 for about 50k.

I ran a borescope up the downpipe and the turbo did not show any signs of seal failures.  Compression and leakdown were great.  The cylinder walls looked fine and the piston crown did not indicate poor ring performance.

After some hard driving the issue went away.  Of course I had already ordered a set of valvestem seals at that point.  Here's what the cyl 5 plug looked like, with cyl 6 on the right.

Yeah, 2000 S4, with K04 hybrids.

Since it's 115+ degrees here for a few months every year I usually park the car for a while and don't drive it.  Before that I did a bunch of cold start and idle tuning, which is horrendous for the cats.

We had about a month of 80 degree weather, so I pulled the car out of storage and noticed it had a slight lifter tick.  I took it out and the ticking went away after a few minutes of transient loads.  I did some data logging and noticed my EGT's were way high under WOT, so I removed the cats figuring they were toast.  Immediately it started smoking on startup, and on engine decel.  The cats were burning all of that oil off.  The amount of smoke was embarrassing and I figured the engine was toast.

That's when I checked the plugs and noticed 6 was oil fouled.  Compression and leakdown were fine, and there was no internal damage I could see.  I had about 6-7 track days on the oil so I changed it, threw a new plug in cyl 6, and went and ran the hell out of it.  It took about 100 miles of hard driving, but it cleared up and it does not smoke any longer.  I make sure to drive it once a week now.

I did order new seals, and let me tell you they are extremely small - so its possible that over time the seal's tensioning springs relax, or the seals themselves dry out and shrink.

I wish I had taken video of it when it was at its worst.  Here it is after it started clearing up.

http://www.youtube.com/watch?v=FHpfCE6eQRs

Yeah, when I'd do a third gear pull and lift, it would literally lay down a thick fog of smoke.

Here's what the valvestem seals look like... Next to an M&M.

http://i.imgur.com/e578R.jpg