I think I can answer this question. I replaced the old 8 speed 105 groupset on my Lemond with tiagra. For comparison, I have DA7800 on my giant.
In terms of ergonomics and shifting quality there is no real difference, except that the downshifts on the front are noisier with the tiagra.
The DA crank is lighter, and I think the brakes may be a bit stiffer, but given the price the tiagra group is great value.