After seeing the turbo inlet pipe or elbow being mentioned again and again I need to get this of my chest (no offence
@aussieR 
)
I know my opinion will not be popular but I would save some money and keep the stock turbo inlet elbow, here's why:
First, I allow myself to use the below arguments for both turbo inlet pipes with turbo inlet elbows and just turbo inlet elbows. Just changing the OEM pipe and keeping the OEM elbow haven't shown to change anything, which shows it all comes down to the turbo inlet elbow alone.
Second, this guy at golfmk7.com pretty much sums it up and he's not even a mechanical engineer:
http://www.golfmk7.com/forums/showpost.php?p=489810&postcount=333
Third, you should understand the behaviour of gasses (air) inside a tube before replacing any part just for the sake of it being bigger. And that's how they make you buy the part in the first place – “it’s bigger, therefore better”. Seeing from an engineering perspective this is not always true.
For comparisons here is the CTS turbo inlet elbow (left) and the OEM inlet elbow (right).
Let’s take a look at the below Large eddy simulations (LES) of your new turbo inlet elbow from TT, APR, CTS, VWR, you name it. There is no need for me to explain this in depth, just look at the velocity and especially the shape where the velocity is highest, see any similarity in shape to the OEM inlet elbow? By reducing the diameter gradually during the 0° to 90° bend we forces the air on the inside of the elbow with the lowest velocity up in speed making the air entering the turbo having the highest speed possible.
One thing is speed but what about the “quality” of the air entering the turbo? We want this air to be as laminar or as less turbulent as possible. Let’s look at the next picture with turbulence in mind. Again, just look at the shape of the lines on the black/white picture. We don’t want the lines to break or change direction during the 90° bend which is what an aftermarket inlet elbow makes room for.
Source:
https://www.sciencedirect.com/science/article/pii/S0142727X08000337
Now you ask me: “So tell me, why does my aftermarket inlet elbow make 5-7hp more than the stock one on my stage 1 car???”. I have no scientific paper explaining this (obviously) but let me try to explain this as an engineer. The stock inlet pipe is designed specifically for the stock power and the stock air amount the turbo will ask for to power the engine. By increasing the boost and the need for air the stock inlet pipe may be a slight bottleneck. But hey, did I just admit that an aftermarket inlet elbow does make a difference? Yes I did, but only WOT (as on a dyno) where the amount of air entering the turbo is limited by the stock inlet pipe and not the aftermarket. During any other scenario the stock inlet elbow will provide better and smoother (more laminar) airflow for the turbo which makes your turbo happy and less likely to f*ck up (which will f*ck up your wallet) and also make the driving experience more smooth at part throttle (someone using an aftermarket turbo inlet elbow actually described the ride to be less smooth on part throttle in another thread/forum, I will find it if you ask me to).
And don’t even get me started on how increasing the diameter of the pipe (just before the turbo) to be larger than the inlet hole in the turbo will change anything other than the amount of turbulent airflow.
(Please remember this is my and only my opinion, if you’re happy with your inlet pipe/elbow I’m happy. If anyone buys an inlet pipe/elbow after reading this I’m still happy, because then the decision was made on calculated scientific pros and cons).
