Have you still got your Boxster S PDK? And if so, has the judder changed?
I've just picked up a 4 year old 911 997.2 Targa 4S with PDK and have a similar 1st gear judder at very slow speed. It may well be normal, but it does concern me. I only tested one other, but I don't recall it juddering. I've read on another forum that the cure is an "Italian Tuneup", ie, thrash it for a few miles and do some standing starts to smooth off the clutch! I'm tempted but even more concerned that it will make it even worse!