I fitted the official VW Helix Plug & Play Sound Upgrade to my MK7 GTI.
I did much research before going for this, and found only really positive reviews about this kit. Helix is apparently a very well respected German audio company, and those that know their music have concluded that this is a great setup!
Installation took about 2 hours, and by purchasing a extended plastic retaining bolt (about £12.00), it is possible to keep the spare wheel.
The difference is nothing short of remarkable. I don't have much to compare it to except for the standard setup, and the Bose system I recently had in an Audi A1, but to say that I am pleased would be a significant understatement! A switch can be purchased (under £20 from VW), which enables a selection between standard and Helix. The amplifier and processor also powers the existing speakers, the result being a very clear crisp sound, with the front speakers only needing to deal with the mid bass.
The only downside was the need to purchase a programming dongle from VW to set select the correct vehicle, as it has pre-set settings. In the States a forum member purchased on and then rents it out for a small fee, but I couldn't find anyone in the UK offering this. It is only required for about 30 seconds to set! Happy to loan out if anyone needs it.
Anyway, any questions please let me know. As you can guess, I highly recommend this kit!
http://www.volkswagen-accessories.com/en/product-ranges/infotainment/plug-and-play-soundsystem.html