I don't think anyone has a US tune mk6 TDI chip yet. I heard rocketchip, alligator these guys: http://dieselinside.com/, and revo have been working on mk6 tunes but I've only seen mk5 TDI tunes. Installation of any chip won't void the warranty on the car but on the engine or anything else the dealer thinks it may have damaged.
You can install it at any time. However, I would wait until the engine is broken in a little longer and for the chip tunes to get sorted out. There are no long term results with chips on the 09-10 generation of cars. The Euro cars were different so the tunes cannot be exactly the same. While I don't believe that the tuners can't work out the differences, no matter how safe the tunes are, they are not time tested yet.
They will have a hard time detecting a tune. I know that BMW ECUs will know if they have been altered. I don't think VW ECUs have anything like this. To the best of my understanding it won't "void" the warranty but they can deny warranty claims on anything if they believe your modification led to the problem. This includes engine, exhaust, and drivetrain, so pretty much all the expensive parts. If they see obvious modifications the dealer has the option of making a note in the service history. For example, IIRC, someone said they had a boost gauge but no chip and the dealer noted that their car was chipped because they believed it was.
So to answer your question, there isn't feedback on US spec mk6 TDIs because people have only just gotten them.
Rocket chip doesn't have a tune. Check out Diesel Power , they were among the first to have a TDI tune and it's a plug-in box, when you go to the dealer you remove it. Gains with the chip are a little better but not easily reversible.
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