Salty, you nailed it. A boat that has a soft (or rounded) chine will slide sideways when put into a hard turn at planing speeds. It also will be very tippy at hull speeds or when it's stopped.
Dave, the company that made the first two hulls has been around for a long time and has produced thousands of "bigger boats" (I'll use that term to describe cruiers as opposed to runabouts). Their hulls aren't "overly wide" for the length, they just have chosen to use a very rounded chine.
Your statement about people who buy the soft chine boats desiring to spend more time at hull speeds may be partially true. I think the fact is that most people don't understand how chines work and the effect they can have on how a boat rides. Mosts cruiser buyers look at a few things (interior layout, space, and cost) and don't consider the hull form.
The aftermarket companies that make the hull extensions have attacked the problem from two directions. The first by lengthening the hull, the second by changing the chine form. A person who wants a bit more hull speed and a soft ride might do better for the first type of modification. The owner who wants to run on plane more frequently would probably do better to get the second type where they change the chine form.
I drew up some hull designs and will give a couple of pro's and cons to them.
A. Flat Bottom. Usually found on small fishing boats
pro--great in shallow water, very stable when stopped or at slow speeds, turns well
con--noisy at planing speeds, hard ride in any chop
B. Rounded (or soft) Chine with flat bottom.
pro--quiet, soft ride
con--tippy at all speeds, slides in a turn at planing speed, hard ride due to the flat bottom. Tends to slap in a chop.
C. Round Chine with V bottom
pro--better ride than hull B, but handles chop better than B
con--Still very tippy, less flotation at planing speeds than a hard chine.
D. Hard Chine with Deep V Hull
pro--good ride in chop, turns better than a soft chine boat, better flotation than a soft chine boat.
con--the deeper the "V", the more tippy they are when stopped or at hull speeds, they tend to wander at idle speeds
E. Reverse Chine
pro--better floatation at planing speeds, holds better in a turn, more stable when stopped
con--tends to slap more at planing speeds, noisier than a hard chine boat. If the chines extend to near the bow it can be very noisy when stopped due to waves being trapped in the chine.
F. Cathedral Hull--this is the classic Boston Whaler hull.
pro--very stable when stopped or at low speeds
con--hard ride in a chop when at planing speeds
Too get and idea of how a hull rides, picture it moving through the water at planing speeds. We're all familiar with running on plane and seeing the water that squooshes (is that a word???) out to the sides of the boat. The more water that squooshes out to the side, the less flotation there is so the hull rides deeper in the water, and that's less efficient when on plane.
A. A flat bottom boat won't have much water going out the sides at planing speed due to the excellent flotation of the hull.
B & C. A rounded chine boat has quite a bit of water going out to the sides because the hull doesn't have good flotation and rides lower in the water. This is less efficient at planing speeds than a hard chine boat.
D & E. Hard chine and reverse chine boats have better flotation than the rounded chines so they ride higher in the water and have less water going out to the sides. They're more efficient at planing speeds but at the cost of a stiffer ride than a soft chine.
F. This hull has very little water going out to the sides because it gets trapped beneath the hull. That's good for flotation but gives a horrible ride in a chop.
Because of the lower lifing ability of a rounded chine boat and the softer ride they provide, they tend to be better for boats that run at displacement speeds where lift isn't needed.