Quickly growing membership can indeed endanger the quality of the discourse and lower the s/n ratio; an important question is how quickly is too quickly. I suppose there are lots of rational people out there who are unaware of LW from whose membership the community may profit.

If the high quality is a mere result of high average rationality level of current members, it would be sufficient to filter newcomers according to their rationality. That can be done by karma system and the growth can be still pretty quick (I assume somebody repeatedly voted to negative total would lose interest in matter of days.)

If, on the other hand, the main reason for the high s/n ratio lies in some surplus quality which isn't simple result of members' individual rationality, but rather set of customs, unwritten laws, atmosphere, or hard to describe "spirit" of the community, then the acceptable growth rates would be much slower, as new members have to be acclimatised before they become a majority.

Then we need data of course. Are there any accessible membership statistics showing a systematically increasing rate of new registrations? Is there a real danger, or is the influx of new members only a temporary phenomenon? And why do you think jimrandomh's proposal to broaden the scope of LW implies larger membership increase? (I also think so, but I would like to hear your reasons.)

Anyway, you seem to have some ideas about how to preserve the LW's quality; perhaps you could write some summary thereof either as a comment here or on the top level.