Canon 8-15mm and Nauticam 8.5" Acryl vs 140mm glass

Are the optical differences between the two dome ports recommended by Nauticam (Nauticam 8.5" Acryl and 140mm glass)  for the Canon 8-15mm immediately apparent or do you have to look very closely at the corners?

I am primarily concerned with the use of the Canon 8-15mm with the Kenko 1.4x teleconverter.

PS: Camera is a Canon R5 

looking forward to your experiences  

It is a bit of a variable feast as it depends on what you are wanting to shoot and the environmental conditions. In general, I think 140mm is a bit small for use with full frame cameras.  My guess is that Nauticam list it as it is a great tool for close focus wide angle images, as it allows you to (a) position the lens close to the subjects and (b) get your lighting in close. It would not be my choice for general wide angle use due to its compromised corne rperformance.

That said, using the 8-15mm with a TC is often used for CFWA, so if this is your goal, the 140mm dome would eb a good choice. 

An 8.5" dome is probably bigger than what is needed with this combination to get great corners. I would also investigate options around 6.5" to 7". Your local dealer will be able to advise on this.  Big domes are great for splits though!

As a general rule, the bigger the dome, the better the corner performance. So logically, if you want to maximise the performance of your high resolution camera, big domes will always be better. However against this needs to be balanced techniques, travel and costs. In your situation, I would buy a 6.5" to 7" dome and add a smaller one for CFWA.

Thanks for your detailed feedback. 
I am currently using the Canon 15-35mm and the 8.5" dome and I am actually quite happy with it. 
The fisheye is not necessarily the look which I like, but with the teleconverter I wanted to give it another chance. CFWA is not my goal, but simply more interesting wide-angle perspectives than with the 15-35mm. 

In conclusion; the 8.5" should not necessarily affect the quality in a negative way or?