105 5700 10 speed and Ultegra 6800 11 speed are worlds apart in terms of shifting quality and speed.
I have a bike with each on and the newer Ultegra bike was noticeably different immediately. As above, I thought this could be the placebo effect, as the kit was all new. Having now done about 20% more miles on the Ultegra, and having changed cables and chains on both, the Ultegra is still better in terms of shifting quality and speed. No question about it.
Difficult to quantify in real world terms, but a lot more enjoyable to ride.
The new front dérailleur of the 11 speed shimano kit is hugely better as an example. I assume this carries forward to the 105 5800 also, but no experience there, sorry.
As suggested already, I would be tempted to maybe look at the brand new 105 5800 groupsan and enjoy fitting myself.