The 140 Suzuki weight is 410 lbs and the 150 is 474 lbs. That is 128 lbs more for two of the 150's. The Yahama 115 weight is 402 lbs, little different than the Suzuki 140, so the boat should be able to handle the weight. I don't know if the boat is engineered for the HP of the 140, but my best guess is it is. There is a suggestion in one article that the boat had been run with 150 E tecs.
The Armstrong bracket only gives a small amount of extra floatation at low speeds or at rest. I believe that the running surface of the hulls on the 255 is longer (the hull is extended) by about 1.5 feet--but I have not measured this. The 255 has the full solid transom--the 24 has motor wells which are on the sponsoons, but do intrude into the usable cockpit area--so that the cockpit is larger on the 255. The longer running surface gives more support aft (It is not at all unusual to extend molds--many 30 foot boats have been stretched to as much as 38 feet) Basically the 255 is a slightly larger boat, with the same beam and some hull extension, as well as the bracket.
My understanding is that an outboard bracket will give better performance, with slightly better speed and fuel economy--it puts the engine in cleaner water and allows the engine to sit several inches higher, decreasing lower unit drag. There is some floatation of the bracket when the boat is not at a plane.
It also allows a solid transom of full height
I don't see any advantage to bracket/brackets on a 24--you cannot really increase the cockpit size easily. I had considered a bracket on my 25, and If I had more time I probably would have done it--in that case I would have eliminated the spashwell, and extended the cockpit, with a full transom (and put in different cockpit floor hatches!) I would have had to do some re-engineering of the entire transom, but at that stage it would have been fairly easy. Often brackets are used to convert I/O boats to outboard boats--and this is a very good conversion.
