Although the first orbit calculation on the MPEC this morning is based on ~2 weeks, thanks to a set of pre-discovery observations, it's certainly concentrated towards the last few days. The uncertainty in the date of perihelion is still perhaps a fortnight or so, not to mention the uncertainty in some other parameters - this happen with Elenin, where the data was jumping around a bit for a while.
With the magnitude parameters from Horizons, while they up the absolute magnitude by 2 (which is a nice estimate for visual vs the CCD data available), it assumes a brightening rate for a long-period comet rather than a first-time comet, and that's not known yet.
Hence my first post
Greg