I think there is more to it than just those two things, and I'll try to explain: With the spring and shock mounted on top of the upper arm, and the upper arm mounted halfway up the shock tower, all of the forces are acting on the shock tower between the upper arm and the top of the shock tower. This results in a lever arm of several inches above the frame rail, where ALL the forces act on the upper half of the shock tower and in just two directions (up and inboard... no wonder they crack). In addition, because the shock is mounted near vertically and the upper arm is both short and the shock is halfway down it, it goes through a relatively large arc for a given amount of vertical travel, so the shock gets into bind precisely because the rubber bushings resist that motion, but just eliminating them will not result in the same motion or the same transmission of loads.
In comparison, the lower arm is much longer and therefore travels through a much smaller arc for the same amount of vertical travel, more closely approximating linear motion. Virtually all the dirt and circle track cars based on street cars use this design layout (where the rules permit). Placing the shock at the outboard end of the lower arm allows a lower spring rate to control the motion, and it can most certainly be felt. With the more linear motion of the lower arm, the lower spring rate and the rod end to allow free movement, the motion is predictable and smooth.
Additionally, some portion of the load is now carried at the
LCA pick up below the frame rail and exerting a force acting in opposition to the force at the top. Imagine if you push in below the frame rail, the rail becomes a pivot point, pushing the top out. The load is distributed all the way to the top of the shock tower, so there are opposing forces (although not equal in magnitude) that help stabilize the shock tower. This reduces the flexing of the shocktower.
Without putting strain gages on both a C/O
suspension and a "stock" layout for comparison I can't say how much difference can be seen in the loads, but if you're just dying to get some numbers, get a freshman college physics book and draw all of the force vectors and make some baseline assumptions (e.g. weight, acceleration, coefficient of friction for the tires etc) and you'll get some idea.
As for me, after I drove the new
suspension I don't really care what the numbers turn out to be... it's a huge improvement I can feel. Not everyone wants to go fast through turns though, and if you drive slow enough and mostly display your pride and joy at car shows, I'm sure the stock
suspension is just fine.
As a side note, most of the improvement of the C/O mounted on top of the arm will be realized by the bearing on the bottom where the arc of the upper arm connects to the vertical motion of the shock. The top can utilize a rubber or poly bushing and provide some isolation. This is the exact same combination in the roller spring perches/stock shock layout that Open Tracker advocates.
Sorry for the tome.