Joe,
The Shimano 105 has often been the sweet spot betweed dollars and performance. It is also often called the "entry level racer". Shimano 105 has decent performance. You get lighter stuff as you go to ultegra and dura-ace but I am not sure that you get anything more than that. Maybe you get crisper shifting. You do pay a considerable amount for the loss in weight.
Now Tiagra is perfectly good too. I used to ride in an "A" group. Most of the riders rode Dura Ace. It was funny that the strongest rider in the bunch was on the cheapest bike (tiagra). I remember one day, the group had been riding hard often up minor grades in the mid 20s for about 30 miles. So we had slowed down to 19 and everyone was recovering. This guy looks at everyone and says "It has been nice riding with you but I need to get my heart rate up". Off he went at what looked like 30 mph. He quickly dissappeared in the distance and the group never did catch him.