A larger intercooler will flow more air, but if your stock turbo struggles to push more air, then it is a wasted effort pressurizing all that extra space.

 

I have a 3.5 ETS with short route piping, I did notice a little difference in throtle response I guess because I went short route, but again like everyone said, whatever you plan on making power wise in the future determines what size you would want to go with, 3.5 is good till I think 800 ETS claims which is why I went with that because id like to reach that one day, my car is also currently on the stock 8.5 turbo. When you do buy one though, don't cheap out on a part like this, quality will payoff in the long run. Good luck
-Chris