My car still has a rough start-up issue, but I too have noticed a difference over the last few months:
If the temperature outside is 2 degrees or below, the car catches and starts very well - it may crank a few times before turning over but it catches very smoothly. For instance this morning it was around 1.5 degrees, I turned the key and it fired up straight away and was as smooth as you would expect.
If the temperature is above 2 degrees then I get a rough clunk as it starts - I can improve this sometimes by holding the key on the starter, but often this can also feel as though the starter is dragging the engine back slightly so doesn't always work so as a consequence doesn't start very smoothly.
I started to wonder if this was an issue with how the ECU was reading the outside temperature for cold starts (the colder it is the easier the first start is) - could the fuel pressure be a factor though with the cold. I don't know anymore but all I do know is I got myself into such a knot with worry over this I just learned to live with it now, as overall the car is such a pleasure to drive (but costing me a fortune
)
I'm still running the car with 10W40 in it, and it's still on original plugs but I will be due it's 4 yr service at the beginning of March (replacing the plugs) even though VW now say they're not due until the car is 6yrs old!!!
Please let me know how you get on.