Should I clean up my old table statistics? I've been cleaning up my indexes but now realized I have over 150 statistics that remain that are not used.  Should I drop them?  I'm not noticing any performance issues from them.  I have 9 indexes and 159 statistics.
Check out this article. 
I think it will answer your questions and give you the tools to deal with over-inflated number of auto-statistics.
The author suggests dropping ALL auto-created stats occasionally, as the one that are required will be recreated when the first query that needs it is executed. It explains the risks / benefits.