Regardless of what you wear to protect yourself from the rain, when you walk, you WILL perspire...a lot. You will become wet with your own perspiration even if your rain gear prevents you from being rained on. Parkas with pit zips help, but do not eliminate the problem.
In this instance, do you REALLY want to be wearing down? Down makes sense if you are walking in very cold but dry weather, or perhaps in the evening if there is no heat in an albergue. But, elsewise, you may be sorry you relied on down to keep you warm when it gets wet.
This is why most of us veterans, wear either a microfiber fleece or merino wool as our "keep warm layer." Both will insulate when wet, and dry out fast. Both do not stink after repeated wearings. Both can be hand-washed, wrung out, and line dried as the need occurs.
I only advise down if it is in a sleeping bag, where it can be kept scrupulously dry at all times.
In most years, I walk during April and May. I usually wear a light, short sleeve, t-shirt or polo shirt. If I am cold, I put my Columbia zip-up fleece jacket on (level 100 is adequate, and level 200 fleece is more than enough). Level 300 fleece is too hot and bulky, IMHO. Carry a brightly (neon - safety) colored microfiber watch cap and runner's gloves to keep hands and heat warm as needed. I get these items in a runner's store.
Read the product descriptions. The better brands will tell you the level, wither in the description or the style name (e.g. Adventure 100...or Stay Warm 200...)
Over that, on a windy day, or if the rain is a sporadic drizzle, I wear my Marmot parka with pit-zips. I wear my sun hat as the larger brim keeps the light precipitation off my eyeglasses.
Once the rain is sustained and constant, even as a drizzle, I KNOW I will be getting wet from rain. So, then I add my Sea to Summit Siliconized Nylon 'body condom' poncho to everything. Also, I switch my sun hat for a ball cap, as this prevents excess water from running around my head and down my back, under the poncho hood.
This all ensures my rucksack stays dry, and prevents more rain from getting in and soaking me. Rain from the outside is also COLD. Cold is bad.
However, all this layering ensures that my perspiration remains close to my body. When I get where I am going for the day, everything needs to be dried.
One final thought and truism to leave you with: "Warm and wet is not bad, and is sometimes very good... However cold and wet is NEVER good..." You might be uncomfortable while sweating, but it beats hypothermia.
Hope this helps.
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