I was at Autobacs Ubi to install my strut bar yesterday. Was chatting with this guy - Ah Yuan - regarding the stiffeners as I am also considering installing after reading the posting in the forum. This was what he told me...
Size B is "bigger" than C apparently. B will fit just nice on the springs after pushing up a little, while C fits easily without jacking the springs. As a result, B in a way stiffens the springs more than C. Cos for you to experience the effect, the springs will have to be compressed if you install Size C. Size B allows little movement. So if you install size B, your ride in general will be bumpier. For size C, it will only cushion the impact when u go over humps.
In the end i did not install. Cos from Ah Yuan, since i seldom carry full load, no CNG tank installed, and I'm not so comfy with bumpier ride (in exchange for more stability), he advised me to delay until i actually experienced the bumper hitting.

That's some sharing for the bros and sis here regarding the stiffeners...

btw size C is out of stock. only size b left. according to ah yuan, new stocks will only come in in Feb...