I just ordered these today - free shipping to most countries.
http://www.ebay.com/itm/161384748580?_trksid=p2060778.m2749.l2649&ssPageName=STRK:MEBIDX:IT
H21W is the bulb type and Bay9s refers to the offset on the bulb itself. Check out that link above and scroll through the pictures, it explains the different offsets in picture form. For the GTI with LED tails, (EURO) we need H21W bulbs with a Bay9s offset.
I too have the Euro LED tails.
So the bulbs above are perfect fitment and error free. And the projector LED beam + additional side leds will provide a lot of light.
Compare to these that will give off a lot less light:
http://www.ebay.co.uk/itm/H21W-BAY9...arts_Vehicles_CarParts_SM&hash=item20db81ef5d
If porshce, audi, mercedes, cadillac, gmc...are all using LED's in their new cars, there has to be good reason. A point to note is that the led systems they use are multiple bulbs much like the bulbs inside our LED tails(the red part), as compared to the single bulb we can fit on our car(reverse).
This bulb is my final change to a complete LED conversion. I've replaced the vanity mirror lights with philips led festoon bulbs, glove box, trunk (massive led with 25 leds), and the cornering lights in the headlamps with philips diamond vision ultimate white light (not LED but still...white). Cannot stand any halogen/amber lights on a car that has Led's almost throughout.