eml....GREAT images...especially love the one of your son....fantastic composition, focus point...it has it all!!!!
I was not aware of the IQ (sharpness) differences between the old Canon 15mm f/2.8 and the new Canon 8-15mm Zoom @ 15mm, (but you know your stuff so I am not questioning your observation).Since the new zoom is 24 years newer...I guess it should have better IQ! LOL!
What the 8-15mm brings to the table is weather sealing, more versatility (I have no real interest in making circular images on my FF...just not my thing) and better flare & CA control....
I have to say...I rarely use my 15mm, but do enjoy the process...many times I get it out of the bag and check a scene only to put it away as it is not working for me...one has to choose ones images carefully with that lens. I find the small size and lesser cost are also benefits to me for a lens that is not used that often. ...and I have to say that I find that the images that I do take with the lens to be plenty sharp, even for the age of the design. They are very sharp across the frame. I do need my CA sliders in LR for this lens, too...no doubt...but it cleans up nicely. Cost around $600 on ebay...could be cheaper if you get lucky.
I have noticed that some posts are bringing up the Canon 14mm, and also the Zeiss 15mm (killer lens) and the Canon 17mm TSE (double killer lens)...but, (respectfully) these are all rectilinear lenses and I just don't see how they fit into a "fisheye" discussion...They do not apply for me in this context. If I want a fisheye...I WANT the curvature and distortion. That is the whole point to me when I grab that lens. :-) (now if our discussion was about running fisheye shots thru software to remove distortion, I could see those comparisons to be useful).
I guess it would be nice to have the new 8-15mm zoom on site...but the cost, weight and size (vs. amount of use) deter me from that purchase...although apparently the IQ is better. (Have looked for side-by-side image comparisons of the 15mm f/2.8 vs. Canon 8-15mm f/4L on the web @ 15mm...but am coming up empty