Ah, I'd forgotten about that little bokeh test
The 55/2 and /1.8 are the same lens, the /2 has a stop limiter (somewhere there is a pic of this: it's just a grommet cutting off the edges). There have been discussions whether the /2 was a rejected /1.8 - which doesn't make sense as they'd have to assemble the lens, test it, dis-assemble the reject, insert the grommet, re-assemble, in order to make a lens that sells for less.
Perhaps they could have selected for lens elements - the closer to spec become 1.8's and the rest /2's. But even that's a bit doubtful. The f/2 is a marketing gimmic just as the SP500.
There is a benefit however - the f/2 lens throws away the most aberrant light paths, i.e. the ones further out from the optical center.
This is the reason why in the old days the slower lenses usually were sharper than the fast ones - the trade off being low light ability vs. ultimate sharpness.
In the case of Takumar, this isn't the case. Especially with the SMC process, the 50/1.4 had less light loss/glare flare, and was sharper than the 55mm. I've got a 1969 Modern Photography that unequivocally states the 50 is sharper than the 55:
Quote: Which lens is sharper, the 50mm f/1.4 or 55mm f/1.8 Super-Takumar? Are faster lenes not as sharp as slower lenses at like apertures? -John Yuen, Ann Arbor, Mich.
We find the 50mm f/1.4 Super-Takumar to be sharper at like apertures than the 55mm f/1.8 Super-Takumar, although generally speaking the slower lenses of most manufacturers are usually to be preferred in sharpness.
That out of the way, we know that sharpness alone, once past some minimum, doesn't determine the ability of a lens. In my torture tests, sometimes the 55 is 'better' with specular highlights and veiling glare than the 50 or the 43, sometimes not.
in these sorts of tests, the distances involved v.s. focal length tend to be more important than relative lens behavior.