I have come to the conclusion that a person should really ride the components they are willing to pay for...

The most obvious differences in the Shimano line as noted are the Sora thumb shifter, and the 9-10 speed jump when you move from Tiagra to 105.

I was going to build my bike with Tiagra, but got a good deal on 105 (I do have Tiagra hubs). For me personally, I didn't have enough desire to spend more to get Ultegra, but I wouldn't begrudge anyone that jump, or Dura-Ace. But to be honest, unless someone has a lot of disposable income, or a sponsor, it takes a serious dedication to cycling to make the investment in Dura-Ace.

The estimate in the earlier message of a 5% improvement seems high to me, but I have nothing to back that up but my feeling that it seems high.