Appreciate the whole investigating, but we've done the exact same thing for... well since we first got the humvee ingame in the ut3 version. Vehicle physics, and physics in general cannot be compared, especially in older engines like w3d, and a not as old engine being ue3. Neither use "real" physics, they both use simulated physics that are less computational heavy and thus easier for the engine to handle. And depending on the requirement, simulated physics can be extremely simple (w3d), or a bit more realistic (ue3), granted udk did update vehicle physics a tad bit. Simulated physics are custom mathematically derived equations that are far less complex and have less variables then real world counter parts. Thus the math behind the physics in w3d and ue3 will be completely different. To compare the two is like comparing mario kart vehicle physics to halo 1 vehicle physics. They are VERY different.
You actually can indeed edit vehicle properties in the editor while playing in game, by looking at the vehicle and typing in "editactor trace". This is what we used to dial in vehicle physics. Originally when I brought in the humvee into the ut3 mod, I did exactly what you're doing which is comparing every little detail to from w3d ren. Trust me when I say this, it is not possible to achieve ren's vehicle physics since in udk.
At the same time I would argue that ren's physics aren't all that great. They feel extremely floaty and do not give you the feeling of driving around in a vehicle capable of blowing shit up. I personally like the more grippy more weighty, and more responsive vehicle controls in renx. The only problem with them is that they are entirely server side and thus lag like no tomorrow, and they can climb up 90 degree walls.
I do like the idea of having a more "bouncy" approach to collisions since that forces the vehicle(s) to separate and not adhere to each other like glue. I'm sure we can work that part of it into the physics, but might be at the cost of other elements of the vehicle physics we may not want to remove/edit.
Only way to replicate ren's physics is to create an entirely custom simulated vehicle physics system, which means it wont run natively and hense may be just as expensive to process even if it is less complex. But like I said, I don't agree with replicating ren's vehicle physics 1:1 I think renx vehicles are successful in other means, and the biggest draw back for me is the adhesion and server lag problems. If those two could be addressed then we'd be far more successful with how they operate and feel.