I have not ridden the new Sora, but I have nearly 3,000 miles on the new Tiagra 4700 and have ridden 105 5800 and Ultegra 6800, albeit for fewer miles.
The Tiagra group performs amazingly well. In my opinion, they only real differences from the higher-level groups are the number of cogs, weight, and brakes. The 5800/6800 brakes are noticeably better. But my 4700 brakes are still pretty good and don't really bother me. I find that the rim and brake pad compounds make a big difference, as does maintaining them (sanding the crud off every few hundred miles).
If you're not too concerned with an extra ~1-2 lbs and don't "need" the extra range or tighter spacing that 11-speed would give you, save your money and return the 105. If you are performance minded and have money to burn, swap the parts over and see the differences for yourself. Since you are asking this question, I'm guessing you are more the former.
Originally Posted by
city_cowboy
I'm one of the few people who isn't especially impressed with 105 5800.
I just felt like there was too big of a performance gap between 105 5800 and Ultegra 6800.
I disagree with this. When set up correctly and tuned well, shifting action is -- in my opinion -- very similar.