If in doubt, get an S3/6R one from the dealer which is only £50 (+VAT) more than these ebay examples. You can feel secure in the fact they would have done proper flow analysis and pressure testing on it before putting it anywhere near a vehicle, as well as OEM due diligence on stress/fatigue and cross climate testing.
There is some snootiness surrounding the plastic end tanks but in reality, they contribute very little to the overall cooling capacity on a day to day road car. Audi dropped the all aluminum one for the 2009 MY S3. They wouldn't have done so if the cost/benefit analysis deemed the ally one to be vastly superior. Audi have always gone the extra mile on everything compared to VW (tighter shut lines, ally instead of steel, thicker insulation, etc etc) so it's interesting the 6R and S3 share the same intercooler.
Anyway, I have no doubt the ebay ones work, but to what degree? How much of the air entering and exiting the core is evenly distributed across the entire area? In some cases, the air is only distributed through the bottom half of the cooler because the end tank design is rubbish. The bigger brand coolers put a lot more effort into that. Intercooler volume is another consideration because the bigger they are, the laggier the throttle response will feel.
