10 logical connections is a hard limit (see http://blogs.msdn.com/angelsb/archive/2005/01/13/352718.aspx and http://blogs.msdn.com/angelsb/archive/2004/09/07/226597.aspx for more details). If you go over this limit, it gets expensive from a performance standpoint.
Also see http://technet.microsoft.com/en-us/library/ms345109.aspx#marsins_topic9 for more information about MARS and performance.