Guides were wearing type III (15 to 18 pound flotation) jackets as far back as I can recall, but I have always told private boaters that unless they like spending most of their swim with a periscope, go with a type V or at least a high-float type III. Both are made in relatively few models compared to standard type IIIs.
The MTI high float type III Atlas mentioned above is a solid choice if you want to avoid a collar. If a collar is okay, the MTI Canyon is pretty comfortable too. The third MTI product with a lot of foam is the universal size Explorer type V, but it is far less comfortable and made mostly for commercial peeps rather than private boaters. Extrasport had their HiFloat for a while in a pricey rescue harness version after the basic one was deleted, but I think even that rescue version is long gone.
It all comes down to cubic inches of foam, but yes, a leaky paddle top can also drag you down. That point was driven home to me during a long swim on the Merced River once. There is some degree of falsehood to the notion that low-float type IIIs are always more comfortable. Since many of them are designed to work with a sprayskirt (which comes up high on your belly), they are often require that a lot of foam be jammed in a very short front panel. Therefore the front panel thickness on a kayaker's type III may be the same as a type V vest. The problem with a hardsheller trying to wear a type V is that it will start pulling them up immediately in a roll before they can set up. But in a raft or IK, this obviously is not an issue. I would try one of the NRS or MTI products and see how they work. And unless the foam in your pfd is really cheap plastic-y stuff, no, it should not have degraded in just ten years. It just didn't have enough foam to start with.