Marinaro was (and still is) my favorite author. But you do have to take what he says with a grain of salt.
After all it was he who--in The Ring of the Rise--suggested crossing two front-facing hackle fibers on a house fly pattern, so its legs would be crossed, like the real thing. And then he claimed (with a straight face) it made a real-world difference somehow.
The Compara Dun and Sparkle Dun variant are two of the most productive spring creek and tailwater dun patterns. They have a good profile and the fish take them well. But they don't float well enough to "float long" and especially to float in a faster-water freestone context.
Your foam blobules on either side of a compara dun thorax would indeed create longer floating pattern. But perhaps not so anatomically realistic. Maybe a more realistic way to add flotation to a compara dun would be to (somehow) add legs, rather than foam blobs.
An under-mounted parachute would be great. But not so easy to tie (there are ways).
Perhaps easier than a parachute would be the world famous under-the-thorax Zelon crossbar tuft.
RE> undermounted parachutes. This takes technology. You can lash a bit of Teflon tubing under the thorax, at right angles to the shank. Then you can wind the parachute around that. Then put a drop of CA glue on the fulcrum of the winding point. Then--once the glue is hard--pull out the tubing. Then snip off the thread loop left by the now missing tubing. This is a lot of trouble and I don't really recommend it. But it does work. It is a way to put a parachute on the underisde of any thorax. And that parachute does increase floatation...dramatically. The Zelon crossbar is easier, and it does (I think) have a more realistic appearance than foam blobs. :=))