I guess you would have to define quickest John.
There are 3 K04 cars that did a 12 second quarter mile and all were 2007 Leon Cupra's with stage 2+ code. There are other cars that have shown higher dyno readings than others.
Dan GB probably has the best chance of having the quickest most capable
Leon MK2 in the UK right now with 4 wheel drive and 450bhp. That LCR thats being converted to 4x4 will may be potentially quicker, depending on the turbo/power/ser up it gets.
The green one owned by mygel is the quickest one i have seen on any
video clips.
If we are talking lap times, pretty sure that topic hasnt been covered on here much. I did 59 second laps in my 2010 Blue Leon Cupra at Knockhill and pretty sure MikeTFSi and BigNod have managed good times on some tracks down south. DanGB doesnt hang about on track either.