A added bulb would no doubt make the boat stiffer, by increasing the righting moment substantially when the keel is down. But, at the expense of added weight, which might not matter upwind in a breeze, but would certainly be detrimental downwind in any conditions. Also the rigging spec and structural calculations would need to be carefully checked. You wouldn't want to do a "Hooligan".
Many years ago when boat-hunting I saw an early Jeanneau Sunshine 36 with 200kg of lead added to the bottom of the keel. It had major structural problems with the keel dropping off the bottom of the boat.
The keel ballast on the 275/285 is I believe in the bottom half or so of the keel. Extracting that and hanging it off the bottom of the keel in a nicely-shaped torpedo profiled to fit snugly against the bottom of the boat when the keel is up, seems a commendable approach.
Although I have no statistics to support me, and am chary of criticising the design, the flat plate on the bottom of the 31/325/335 keel looks all wrong to me, although I understand the rationale, in that the boat sits neatly on top of it.
275/30 "Cascada"