I've heard that every 3 years Shimano upgrades their component groups & the lower end groups basically inherit the technology from the higher end stuff.
ie: 2008 105 is equivalent to 2011 Tiagra.
I've used both (2008)105,& Tiagra(2011) on CX bikes,I prefer the 105 much better.
IMO the 105 feels smoother when I shift,Tiagra is clunky sometimes.
Could it be adjustment,probably.
I also like the finish of the 105 over the Tiagra.
Why hasn't anyone mentioned Dura Ace 7800/7900?
I thought we we talking about "high end"?