Rec. for a packable shell jacket that fits comfortably over a thick parka

Are you looking for a waterproof or wind/abrasion shell? I keep going back to a particular wind jacket that is baggy enough to layer over my puffy, which I only wear in camp or at rest.

Why a shell?  The jacket should already be windproof or down would leak through.  I don’t expect it will be raining if you need such a warm jacket.  The jacket is not sewn through, it is baffled, so a shell will not add additional warmth by trapping air at sewn through seams.

I don’t know about the OP, but I bring a wind jacket anyway, so just opt for the baggy one as a little protection for the puffy from tree branches or a few minutes of rain/wet snow.

That wind jacket would have to be pretty big in this case. Did you see how big Holger’s new parks is?

It also has 20d face fabric with 30d in high wear areas. I’ve found 20d wind shirt fabric to be quite tough enough for most things, certainly against branches and the like. I think this parka is well-designed to function without anything over the top of it in all but the most severe conditions. (Antarctica, Everest)

Well, Holger is going into really frigid conditions. I’d take a shell. Cold winds are nasty.  Shells really help with heat loss, as a rule. A lot.

Are we talking about human powered use, where weight and size matters?

I would want to look at the jacket in question carefully. It it truly box baffled everywhere? And how bulky is it? How many seams(looks like many).

Assuming you are bringing a shell anyway, then indeed, having one that is big enough to fit over the insulation jacket is certainly the best  warmth:weight option.

However, if there are no thin spots in the parka, then the warmth gain in stormy weather will be minimal, and the loss of functionality from a giant, flappy oversized  shell jacket the rest of the time might be to much to accept. This depends on your activities too of course. Alpine climbing will not tolerate much of that, while smooth ground walking or skiing would.

Either way, I would certainly not bring a separate, second, shell just to go over the top of my parka.
Besides the poor weight:warmth ratio (3.5 oz of extra down will be much warmer than a shell over the top of a fully box baffled parka like this), there is the klutz factor.

When it is really cold, I have my head covered by a balaclava, and use collars and neck gators and hoods. My hands are in liner gloves inside big mittens. All this means that adjusting and adding layers and zippers is much harder than in mild conditions. So every extra layer, with its own zipper and hood, bockes somehting to sort out and deal with.

I know it sounds silly, but it’s a real issue. So simplicity rules.

If you are worried about being warm enough, and a shell jacket sized  for both active use and over down parka is to billowy and unwieldy  for you, I would add an insulation layer, like a Polartec Alpha sweater or a down vest,