Linguistic note: "It's a Canon L-series and should be giving you excellent IQ," is a statement with two separate clauses, not a dependent clause, as in, "It's a Canon L-series and so should be giving you excellent IQ."
You are supposed to be a surgeon or something like that, not a linguist or otherwise I wouldn't have dared to comment on your comment :-> ... though it's good you clarified this because some time ago I was under the impression all L lenses have to have top iq, and most salesmen offering them would tell the same :-p
The 24-105L is a very good lens, espeically stopped down to f/8 which the OP mentions.
No doubt, but as far as I have read about it you can still get softer results because the IS is older and it's not sharpest wide open (the latter like my 70-300L, 17-40L) - I mentioned this because the op's shot was wide open with strong bokeh.
I think this thread brings out a simple painful? truth. That is electronics have developed to the point, that the overwhelming advantage the FF cameras had some years back - is rapidly closing down.
But this is just because the 5d3 and 6d have failed to make any significant sensor update - ff has much more potential left for higher mp (look at the Nikon d800), so if your lenses can keep up ff has a bright future for many types of photography.