What a difference a couple versions make...I had been using a slightly older version of Lucene.Net from:

 http://incubator.apache.org/lucene.net/download/

After getting the latest SVN version and applying this patch to fix the IdexOutOfRangeException during indexing my Documents/second measurement went from 400 (after experimenting with various buffer settings) to 12,000 using just the default settings! I can index my data in a fraction of the time.