Late on this. I just elaborated on my impressions of my K04:
I was meaning to write a review on this actually... here are my thoughts:
Initial Start up:
The cold-start warm up is very different on the APR K04 file. It is MUCH better. The car doesn't sound obnoxious anymore and I'm finally content with my exhaust system. Before, I would HATE my car on cold starts. It was obnoxious and would garner unwanted stares from people near by. This is the way the car should be on warm ups.
Low/calm RPM driving:
Car feels identical to the stock turbo. Even on the K04 stock file, it feels almost identical to stock. I noticed that there were more turbo sounds. You can hear the compressor spooling up, but it only spools to 10psi, so stock-like. You would not notice you're driving a K04 unless you were REALLY looking hard... even then, you may miss it.
Normal driving mixed with spirited driving:
Car feels tamed during the normal driving. I feel like the car actually pulls more linear than stage 2. That being said, I do notice more turbo lag. As others have pointed out, there's nothing you can do about since it's a larger turbo. That being said, it's not a "big turbo" by any means. It is still an OEM turbo and just a smidge larger than the IHI (stock) turbo.
To elaborate on the linear feel, I went from stock to stage 2, so I will talk about stage 2. Stage 2 feels great. My favorite gear was second. As soon as I was at 3k, I punched it and I had full power almost immediately. It was fantastic. It also died off around 4700rpm (power). I felt like I was just waiting for the RPMs to finish so I could shift. With the K04, you punch it at 3k and it takes a tad longer to spool (maybe 1.5s) and you have a much stronger pull. The difference is, the pull lasts through most of the RPM band. Third gear now pulls really hard. Higher gears feel really nice. First and second are laughable.
Spirited Driving:
It will put a smile on your face if you start from a first gear roll. Utilizing APR's hard rev limiter to your advantage and shifting through the gears, you'll be extremely satisfied. Before you know it, you'll be at the top of fourth. It's just really nice having more power throughout more gears. Whereas the stock turbo, you feel the power to start to die off as you go higher in the gears and RPMs.
Sounds:
The stock IHI and OEM K04 have turbo mufflers. APR/AWE/HPA/Revo/CTS all remove this and it results in a K04 fitting the TSI AND more turbo sounds! This sucker spools up to around 20lbs and you can hear it. It sounds awesome. That being said, I also do notice more motor sounds in the cabin as well.
Overall Impressions on Drivability:
It is absolutely drivable for daily driving. I have no issues w/ it at all. I love the power. I love the confidence in the power as well. If I had to give it a con, I'd probably say that second gear is useless. You have to modulate the throttle to control the wheel spin. I get slightly lower mpg than I did on the IHI turbo. Maybe its because my foot has been heavier w/ the new tune and turbo, but I noticed it is a tad lower (maybe 2mpg).
Hope this was helpful.