First, one thing I will say regarding the camera shop: (I don't really care to defend them), but anyway, I did not get the full context of the conversation, so was unable to report it here. But this has triggered a very interesting thread (at least to me).
One thing I have done related to this topic: I have a 70-200 2.8 Ultrasonic L non-IS lens in my possession for a couple weeks.
Yesterday, I went out front and took some pictures with that lens, and my 55-250 F4-5.6 IS lens. I was wanting to compare pictures. So I set up to take the exact same photo, changing lenses to get it again, with both set at f.4, and 70mm, during the afternoon.
I then put the photos on my computer and tried to compare. Zoomed in to look at it extremely close; I compared the exif information. Studied color, and detail.
Frankly, about all I could find for difference was that the L lens, with its larger lens diameter, took the same picture quicker. Did it several times, and found in one example, mine took it in 1/500, the L did it in 1/1000; another time mine 1/1000, the L 1/1250. I"m new enough to this that I thought maybe mine may look a bit grainy in comparison, but that didn't seem to be the case.
I realize of course the quality material differences, but my questions and concerns, at this stage in my photography excursions, is quality of picture.
The one thing for sure I do interest in is getting 2.8 vs 4.0 lens.
At the moment I'm struggling between the 50mm 1.4 vs 1.8, and 1.2. Given the cost of a 1.2, and my very early stages in photography,I won't be getting the 1.2, but am leaning a bit to the 1.4. (Have read several threads here on that topic, and hard to say what's best) And, I have no idea if I would / will ever be good enough to make $ with this, so not sure I want to spend that kind of money.
BTW...I did find one video comparing the 3 lenses, and a site with some discussion and pics, and the 1.2 does definitely seem to provide the best pictures. But at quite a price difference.