Sorry, how are this calculations done? To me, I agree, it's a little brother to a 200 f/2 - if it's sharp enough for the cropping.
Sorry it's late and I just assume everyone knows what I'm thinking..
For example, if you were to use the Canon 135 F2 at a low light reception/wedding, and following the rule of 1/shutter speed, 1/125 to avoid camera shake. So say its dark enough for it to be F2, 1/125, ISO 6400 but with OS (stabilization) and assuming we get a few stops of it, you could be shooting at F2, 1/30 ISO 1600 instead provided the subject is quite stationary. So you could get shots the Canon cannot get.
I said a 'mini' 200mm F2 because if you had the 200mm F2.8 at a dark reception, at 1/200, F2.8, ISO 12800.. but with the F2 having IS and an extra stop of light, you could be shooting at 1/50, F2, ISO 1600 instead.