Overpitching can be a bad thing.
Chris White of White Labs says:
If the beer is overpitched, yeast do not grow though a complete growth cycle. This results in few new yeast cells, which makes for unhealthy yeast and low viability by the end of fermentation.
Thus, if you intend to harvest and repitch, or if the yeast run out of steam and crap out before the intended attenuation is reached, you may be in for problems. Also, as menschmachine noted, the flavor/aroma profile of the yeast may be adversely affected. The reproductive cycle of the yeast is when the esters, phenols, and other flavor precursors are produced.
In my opinion, most homebrewers underpitch. The risks from underpitching are much greater than the risks from overpitching, in extended lag times, weak reproduction, insufficient fermentation power, and
too much ester production.
Here's a bit from beeradvocate.com that states the remaining concerns nicely:
From what I understand, the issue with overpitching has to do with the same biological survival mechanism that brings us attenuation. When yeast reach a point where there is no longer enough food to sustain their numbers they go dormant and wait for more food to be introduced. Now, overpitching becomes an issue if the ratio of cells to food is too low. This will cause incomplete or stalled fermentation as the yeast will drop out of solution and wait for more food. Additionally, they seem to stop reproducing once they reach a point where they can no longer sustain. So say your first beer had a starting gravity of 1.050. If you took a cup of that slurry and repitched it into another wort of equal gravity they will reproduce to roughly the same number of cells as was the final number of cells in the original beer.
As others have pointed out initial cell count will have its greatest effects in terms of chemical byproducts of reproduction, not fermentation itself, such as phenols and esters. This is why many recommend underpitching on hefeweizens; forcing the yeast to reproduce to environmental capacity from a small cell count maximizes the production of such byproducts.
It is best to pitch the appropriate amount of yeast, which is easily learnt by clicking on
Mr Malty Pitching Rate Calculator and entering the correct information.
Your fermentation, J, is fine. Short lag times are a good thing. Rapid fermentations are a good thing. It might take a little while to completely flatline, so give it at least ten days in the primary before you go fiddling about with it.
Cheers,
Bob