Having had experience with APR tuning, and also having just got a used 2017 S3, I figured it'd be worth chiming in
I had a 2015 Golf R w/DSG that I just handed back in (it was a lease). I put both the APR Stage I high torque tune on it (91 octane as that's all I can get here in Los Angeles), as well as the TCU tune before 800 miles were put on the car. After 3 years and 33,000 miles, no issues, and I was as happy as a clam! Before getting into performance details, I wanted to address the warranty question. Yes if you go to a dealer for service, and the tune is currently installed on the car, it will get flagged and your powertrain warranty will be void. Hence what you do is take the car back to your tuner, have them re-flash everything back to stock, and then you go to the dealer to get your warranty service, and the dealer will have no idea that the car was ever tuned in the first place. After the warranty work, take it back to the tuner shop and have them re-flash it. So the main downside to a tune is that if you do end up needing warranty work, you'll have to make 2 additional stops, as well as pay for the flashing (which for the tuner shop I go to would be $50 a flash).
Now of course if you drive your car foolishly, and you go into the shop with some crazy blow out, you'll raise suspicion. But this is no different than tracking your car and taking it back to the shop, and then trying to convince them that whatever happened was in the course of normal driving. A lot of the stories I've heard about warranty or tuning related issues are due to the fact that there are a lot of people that want their golf to beat a Ferrari, and they don't understand the concept of properly upgrading to those levels of performance and what they can do to an engine if not done correctly (i.e. slap a larger turbo and intercooler on it and tune it with nothing else). I daily drove mine, and while I drove it very hard, I wasn't racing it constantly, or running it flat out for hours at a time, or doing launch control sprints at every stop light (I think I did about 10 - 15 LC starts over the course of 3 years). I did the occasional canyon run, as well as zipped around traffic and such, but I never tracked or dragged it. The engine and transmission in these cars are built like a tank.
Also APR has a new program called APR Plus (
www.goaprplus.com) that provides a powertrain warranty for your car with their Stage I tune and bolt ons. Meaning they take over the remaining powertrain warranty for your car, but you can't go above APR Stage I (i.e. no downpipe, or turbos and the like). It's $400 more than the standard Stage I tune (from what I can tell), and comes with the same Stage I HP, but about 18ftlbs of torque less. But you can add their air intake and intercooler as well for a jump, and still be in warranty. This is the route I'm planning on going with the S3
One thing to consider is that it's very much worth it to get the TCU tune as well as it works in tandem with the ECU tune. It also protects the DSG due to the way they tuned the shifting parameters (it's listed on their site if you want to read the technical specifics
As for the performance, it transforms the car! 0-60 in the VW went to about 4 seconds flat, and I've seen videos of cars with Stage I only running mid to low 12's in the quarter mile. There's so much torque and punch that makes the car a beast at just about any speed. Maneuvering around traffic is effortless, and it makes the car an absolute sleeper. It's very easy to drive every day, with no jerkiness or finickiness that one might expect from high performance tuned cars. I miss it like crazy right now, but the financials aren't right just yet as I want to have several things done to the car all at once
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That's about it, hope the information is helpful!