I went for a 300km ride with a buddy yesterday that started out around 10°C and ended up around 16°C. Thankfully, it was very sunny all day.
Nevertheless, I was wearing:
(on my legs)
-underwear
-long johns
-thick rain pants (made for cold-weather running)
-jeans
(on my torso)
-undershirt
-t-shirt
-wool sweater
-armored-leather riding jacket with warm-weather lining
I also had a neck warmer, Winter boots, ski gloves and a full-face helmet.
In the morning, it was a bit cool, but by the afternoon, it was just right. Mind you, when we stopped for lunch, we couldn't peel layers off fast enough. But riding along at 90 km/h on winding country roads, it wasn't overkill at all! Oh how I miss the days of 25°C riding...