In your example, the FF Helios is 53% down (by weight), the down pants are 31% and the booties are 50%, for an overall percentage of 48% (13oz/27oz).
I believe the main reason this isn't commonly employed is because sleeping bags are typically more weight efficient than down clothing - meaning they have the highest proportion of their weight in down. For example, Zpacks 20 degree sleeping bags are 65% down, so you could get the same 13oz of down for 20oz total instead of 27oz.
Now opting for only this sleeping bag wouldn't be a very versatile tactic, since you'd have no insulation for when you're outside of the bag. Accordingly, my strategy is to employ just enough down clothing to comfortably meet my needs around camp, and then put everything else in the sleeping bag. So if I wanted 13oz of down total, I might opt for an 18oz Zpacks bag with 11oz of down, and then a 4oz Black Rock down vest with the last 2oz of down. That sums to 22oz instead of 20oz for the same total amount of down, but it's more versatile. If 13oz of down is going to be enough for sleeping, 2oz on my torso is probably enough for around camp.
Ideally I'd like 3-4oz of down in clothing and another 9-10oz in the bag probably. If you found an 8oz down jacket that was 40-50% down and then got a sleeping bag at 60% efficiency, then you'd wind up around 24-25oz and have a well rounded system. It would probably actually be better than the all clothing system, because a Helios parka with 9oz of down is down is darn warm for wearing around camp in 30-40F temps. I say 30-40F temps, because it seems like we're expecting a nighttime low around 20F if we're only packing 13oz of down total.
Taking this further, going to all clothes and no bag is likely to be the heavier still to get the same amount of down. It is true that some super puffy parka's have pretty high % of down like your Helios, but you're probably not going to get as high as sleeping bags do. If you have really weight efficient down layers like the Helios, then you've got more flexibility between taking more layers and a less warm bag or vice versa, but for more people this is a considerably heavier option.