All,
Don't laugh!!!
I did a basic (non-scientific) volume test - capping the bottom of each manifold and filling with water. The volume of the full 2.5 manifold poured onto the 3.0 with room to spare. Water from the full 3.0 overflowed the 2.5 by quite a bit. (I guess I could have measured the fluid)
The long and short is that it does seem that although the external dimensions are similar, the 3.0 has a higher capacity and I assume flow - and the thinner walls might be more prone to cracking.
As I mentioned in the beginning of the thread, my right side manifold was cracked. Does anyone have a 3.0 spare for the right side? I'm in southeast USA. Thanks in advance.