Had the same problem with 1 cylinder running rough right after startup. I had just put in new glow plugs a year ago. I finally tested the resistance on all the plugs, and sure enough had a bad plug. Easy peazy. I had installed the plug with anti-seize and it was easy to remove and replace. Problem solved.
Yes, this car is fast! No cat with straight pipes. ALDA adjusted 1.5 turns.