There actually is a 47" version of the LK450, but it seems to be only available in Canadian markets
So if the OP is outside Canada, I guess this is a moot point.
As for the LK520, I *vaguely* remember reading somewhere that it had higher input lag than the LK450. But more importantly, that it failed 4:4:4 chroma. So assuming what I read was legit, then the LK520 is out the window for PC monitor usage.
The only other TVs (that I know of) that uses S-IPS panels is the JVC Black Crystal series and Vizio XVT series (last gen, not sure about the current gen). Unfortunately those both fail 4:4:4 chroma also, so those are out the window.
So in a nut shell, if low input lag + S-IPS panel + 4:4:4 chroma support + 47" size are your requirements, I think you'll be hardpressed to find anything that fits all those criteria.
However, if you're willing to budge on the S-IPS requirement, you may want to look at the Sony EX720 series. While not S-IPS, it meets the other three requirements.