I have no idea about the framing accuracy that you asked about, just responding about the Pentax-L 43mm f/1.9 Special. It is a superb lens in every respect: Light weight, fast focusing, beautiful focusing action, and very very nice imaging qualities. I've tested it extensively on both the Leica M-D and Leica CL (digital) bodies, as well as the Leica M4-2, and it is one superb lens.
I suspected it would be because the Pentax-FA 43mm f/1.9 Limited that it shares optics with was without a doubt my absolute favorite Pentax lens out of all of them I owned when I was working primarily with Pentax digital equipment. It was such a good lens that I considered keeping a Pentax body around simply to be able to keep and use that lens.
Its performance on the digital sensor cameras is far better than the Summicron-C 40mm, or either of the M-Rokkor 40mm variants.