Focal and JL audio are not the way forward in these cars...I have done extensive research into what works and what doesnt...BUT, the brands ill suggest will probably be unheard of so you may dismiss them straight away...but believe me when I say they are the best kits out there...
In the Mk5, you generally have a 3 way component kit in the doors, consisting of a tweeter, a midrange speaker, and a mid bass unit. Most people tend to just put a component kit in the door and use an adapter to allow fitment of a 6.5inch midrange in the stock midbass unit location. This is bad on a few counts....the component 6.5inch speaker is not a midbass unit, its a midrange unit. The midrange unit will be firing into the side of your body, when ideally, you want it firing infront of you (down by your legs or in the corners of the door closest to the a-pillar).
The Gladen Audio ONE 200 Golf kit comes with a pair of 8inch mid-woofers, 4inch midrange, and tweeters for OEM fitment and a GREAT sounding OEM upgrade. I have heard reports of people not requiring subwoofers as the front doors sound so good.
To push this further, add a four channel amplifier, a pair of channels can be driving the midrange and the tweeter, and the remaining channel can drive the midwoofers.
I have extensive experience with Gladen Audio and fitting their kits into mainly BMWs, and man, they are simply jaw dropping...I have seen people change from Harmon Kardon, Logic 7, Bose etc to the Gladen kits and not looked back!
When we find the right car, I will be installing the following (its bound to change but a rough idea will look like this)
Gladen ONE 200 Golf 5 kit
iPad mini or some form of Pioneer stereo
Mosconi amplifier for the components
Tru Tech Hammer amplifier for sub
Gladen Audio Zero Pro 12" or 10" subwoofer
Should make for a jaw droppingly sweet sounding system!
If you have a Mk4 Golf R32, then there are also 8" components available for that...
Again, provide AWESOME mid-bass, and if you have a 5 door, then you wont need a subwoofer with 4 8's in the doors!