Travel is full of compromise. Packing an appropriate jacket shouldn't be one of them. Get the comfort and insulation of a down jacket, with the compact packable ability of a shell jacket.
Using an 800 density down you are guaranteed a jacket which is both feather light and super warm.
When packed down it's no bigger than a smartphone - so it'll slip into your rucksack, or stow away in your glove box. It is also fully water resistant with a flattering.