based on my personal experience i would suggest bikeworks down on ridge st. i have found dave, erin, and rob to be real friendly and knowledgable. they've been focusing on track specific stuff longer than just about any shop that i personally know of here in manhattan. they're the kind place where you could probably go in without knowing some secret handshake and literally compare the difference between a 14 & 15 tooth cog by actually trying it on your bike. the best part is that they're totally unpretentious. dave was a professional rider who grew up and rode with people like greg lemond. and erin has won female rider of the year for past two years at kissena's twilight series. meanwhile their there happy to help you with their greasy hands most days out of the week whether you ride a nagasawa brake or no brake or your a chinese food deliverer with huffy mountain bike stuck together with scotch tape.
http://www.bikecult.com/works/index.html

-cheers