Spoke to MRC who know their stuff. They've been the only garage who said they will specifically map the car for me. They also advised that the gear box would need to be mapped to handle the difference in torque. APR told me that the gear box didn't need to be mapped!! Its so ****** confusing as I assumed APR were in bed with Audi so know what they are talking about. Then MRC just turned that on its head
@andyay123 - I apologise if I'm about to insult your intelligence...
Before you tune, be aware of the consequences
Increasing power and torque will increase stress on the engine and other components. As such the consumables of both may "wear" more quickly. This means that it is wise to service a tuned car more frequently, but also to change certain "lifetime" components and consumable to service items. So for example gearbox ATF oil & filter ought to be changed at less than 38K intevals and it would be wise to have then hypoid oil changed at some point. It is also sensible to be aware of the weak points in the car because tuning
might make them fail more quickly. On the B8 the DL501 gearbox is top of this list (the 3rd version fitted from 2103-ish is hopefully OK, but ones before that are at risk). Next is the crank pulley which is likely to fail at some point. On early engines the water pump wears out. On the newer engines (mid-2014 onwards) the supercharger clutch is a risk item. Ideally these risks should be mitigated (where possible) at the time of tuning, but if that's not done owners should be aware that tuning might accelerate failure.
Stage 1 vs stage 2
The OE ECU map restricts peak torque and power by opening a bypass valve in the supercharger to recycle the air and limit manifold pressure. In simple terms stage 1 raises the threshold of when this is opened. More air needs more fuel so the fuel map has to be changed. However depending on the tuner, other maps such as the spark timing can be tweaked. To increase torque and power beyond this, even more airflow is needed to increase manifold pressure. This is done by increasing the revs of the supercharger in relation to the engine revs. The OE pulley ratio is 2.56 so when the engine is at 7,200 rpm the supercharger is at just over 18,400 rpm. One way to change the ratio is to use a smaller supercharger pulley. It has been found that the smallest without risking belt slip is about 57 mm and this increases the ratio to 2.84 and the supercharger revs to just over 20,400 rpm. However the Eaton supercharger is spec'd to 24,000 rpm, so the ratio can be increased to 3.33 (for a 7.200 rpm redline). This can be achieved by increasing the size of the crank pulley, either on its own, or in conjunction with a smaller supercharger drive pulley (which is known as DP - dual pulley).
Stage 3
For completeness it should be mentioned that further gains are possible. An attempt was made at a supercharger upgrade but its benefits were never realised, especially given the price. The other solution is to increase maximum airflow further by removing the restriction at the throttle by increasing its size. I think the only commercial TB upgrade is from APR with their "Ultracharger" brand.
Tuning requirements
I think one of the biggest areas of confusion are about what else needs to be done as part of the tuning. I mentioned components at risk of failure above, and I'm going to ignore subjects like braking and suspension, and focus on things directly related to performance.
FUEL: In basic terms the octane refers to how "smoothly" the fuel burns. An engine which is tuned to do so can extract more energy from a higher octane fuel and this gives better torque. Most tuners program the ECU to do this, and so a tuned engine will have higher torque and power with higher octane fuel. However the engine should still cope with lower octane, it will "******" and run at lower power.
AIR: Increasing the manifold pressure in stage 1 and 2 is only possible if there is sufficient airflow to allow it. I have not seen any evidence, but it would appear that a stage 1 tune is not restricted by the OE intake, and certainly not at the OE redline. However I have seen evidence that stage 2 will be restricted at the top end and that's why intake uprating is always recommended for stage 2 (and a stage 3 larger TB is simply an extension of this uprating).
TRANSMISSION: Increasing the torque from the engine is only of benefit if it can be transmitted to the wheels. By definition the point in the transmission where loss of torque will be highest is at the clutches. The clamping pressure of the clutch packs is set by the TCU. There are many cases where tuning easily exceeds this OE pressure and there is slip (I believe the S3 is such a case). I believe that MRC has also seen it on the S4/5 which is why they recommend the TCU remap which increases the pressure. The other limit in the TCU is the redline. The B8 had this at about 6,800 rpm (I think) whereas the B8.5 was reduced to about 6,250 rpm. It is accepted that this platform can cope with around 7,200 rpm so most TCU remaps will increase the redline to this, releasing a bit more power.
EXHAUST: Unlike many platforms, it seems that the exhaust system on the B8 is not restrictive even at stage 2. However the cats are vulnerable to heat and may be restrictive at stage 3 so ought to be uprated or moved.
HEAT: Since hot air expands heat is an enemy of tuning (particularly forced induction). The heat exchanger system varies between the B8 and B8.5, and again for the CREC engine, so the situation regarding what uprating is needed is unclear. Of course actual running heat relates to how the car is driven. Uprating is recommended in stage 2 for hard driving and hot environments, and seems to be a requirement for stage 3.
Custom tuning
As I noted, any stage needs ECU reprogramming to change the engine maps. Without doubt there is skill to this and some tuners can optimise maps better than others. This has more impact the higher the gains. However also no two cars are absolutely identical, and a skilled tuner can optimise a map for a specific car. Most tuners use generic maps which were created on a development car. Other tuners (MRC being the prime example) will start with their generic maps but then tweak them for each car to create a custom tune which (in most cases) results in slightly higher performance.
). I am not against graduates developing the tunes etc but then don't expect it to be 'perfect'! Also, it's not just the state of the art facilities that matter but the R&D. 
