I have never really been 100% on board with either side on the carbon buildup issue. The two sides I’ve seen (and I am admittedly a pretty black and white thinker, so forgive me for generalising people’s positions on the issue) generally work out as follows:
group 1. carbon buildup is a disaster for the RS4, destroys the power the engine makes, is an all around terrible thing that needs to be cleaned annually, otherwise the car is a shadow of its true self
group 2. carbon buildup is a non-issue not worth worrying about, and has virtually zero impact on the car
So…my view is as follows.
I think it hurts performance - maybe enough to cost you a car length or so in a dragstrip run…which is about a tenth of a second on a timeslip…which is maybe half to one car length in a highway pull.
I think it is a scary foreign object to have inside our engine, so to that end, I think it needs to be cleaned periodically just for engine safety.
I think it hurts the dyno far more than it actually hurts performance. Fortunately, I don’t give a fuck about the dyno so this doesn’t bother me much.
I think RS4 owners need to get over worrying about carbon buildup all day with all the dyno and logs and blah blah blah.
Now that this is out of the way, I have to say BEFORE I owned an RS4, I was pretty squarely leaning towards group 1. I thought carbon hurt an RS4 quite a bit, performance wise. I thought it robbed big hp. I thought the car suffered as soon as 5000 miles after cleaning etc. I used to get into vicious arguments with Arthur who was squarely in group 2. Basically I believed the hype. I shouldn’t have trusted people’s opinions/hype, and should have formed my own opinion.
Now that I actually own an RS4, and have done tons of driving, data logging, dragstrip passes, pbox acceleration testing (and I would submit I’ve done more of the latter two than any RS4 owner on earth)…I have come to realise that it’s really not that big of a deal, performance wise. While it may rob you of power on a dyno sheet…it doesn’t rob you of enough power to really show up on the street. You might feel you’ve lost the kick at 5500 RPM (and I’ll address that later). You might feel you’re not winning races you should be winning. You might run shitty quarter mile times. It’s my belief that all of those results are explainable for almost all of the RS4s. Here are those reasons:
1. "I’ve lost the 5500 RPM kick"This makes the least sense to me. The tune is the tune. The throttle angle is the throttle angle. If the car is choked, the throttle angle is still going to open at 5500 RPM, allowing more air to pass, allowing the car to make more power, allowing you to feel a kick. The stock map restricts the throttle in gears 1-2-3 below 5250 RPM.
If we dig deeper, there’s another factor. Ever notice the kick is far more pronounced in the cold? Why? It’s because the engine’s stock map restricts throttle up until 5250 RPM or so in gears 1, 2 and 3, depending on a number of conditions. It’s my guestimation that it bases this on ‘engine torque’ made. Make too much (for its parameters) and it puts a bigger restriction on. Make not much torque (relatively) and it eliminates the restriction.
ON a cold day with dense air, the car makes more power/more torque and thus the ECU restricts throttle quite a bit. Around 52% even if you go WOT with your right foot.
On a mild-warm day, the car is making less power (torque) so the ECU allows the throttle to opena bit more…I’ve logged it in the 72% range a few times.
On a hot day with a heatsoaked car, the car is making far less power (torque) so the ECU allows the throttle to open 100% in those normally restricted gears in the normally restricted RPM range.
So there’s not always a 5500 RPM kick to be had. Sure the cam profile changes etc. etc…but the throttle restriction is the biggest reason for that kick…and the temperature/altitude/density has enough influence to eliminate the throttle restriction altogether in those gears…so when someone from Florida posts up that they’ve ‘lost the kick’…there’s a good chance they could go a full year without feeling it because their hot temperatures might mean the engine never has to implement the protective throttle restriction.
I have logs of all of this in case anyone wants to see it.
2. “I ran shitty quarter mile times” or "I lost a race I shouldn’t lose"People love to compare their quarter mile times/cars to the magazines etc. Quite often, people fall short of the 12.8-13.0 @ 107-108 times we saw from Road and Track, Car and Driver, Motor Trend, Insideline…etc. There are two huge reasons for this. One is that people tend to go to the dragstrip in the summer. As we know, in the summer the air is less dense. As we know the less dense air means the car is making less power. So as a result, if you hit the dragstrip on a hot summer night, and sit around in line waiting to run your nicely heatsoaked car, guess what…you’re going to run a shitty time because you’re no longer making the car’s full potential power. Further, those magazines you’re comparing to ALL correct their times to 0 density altitude (and a humid, 25 celsius night at a dragstrip at 800 feet elevation above sea level is going to net out around 2000 feet of density altitude).
The second reason people run shitty quarter mile times is simple - they aren’t very good at it. They stage wrong. They launch wrong. They shift slow. They bounce off the rev limiter forgetting to change gears on time. I have seen a ton of RS4 videos of guys driving both on the dragstrip,and in exhaust/cruise videos etc. and plenty of them frankly leave performance on the table. We shouldn’t be surprised when they do this at the dragstrip. People also tend to go to the strip on a busy night…get maybe 2-3 runs in…then leave discouraged by all the waiting (and the shitty results). This is a pretty key factor because it takes a few passes to figure it out for most people.
Obviously this is just my opinion, as someone who has owned a ‘carboned up’ Rs4 for a year now. I will buy a carbonator if it is ever made available, because as I said above, the ‘foreign body’ in the engine doesn’t sit too well with me. If it preserves that last few hp carbon would otherwise rob, that’s a bonus to me.
I would like to hear input from other folks. Anything but dyno numbers. 
I’ll make a new post introducing myself.