Well from my experience at a Bortle class 4 site, anything wider than a 4mm exit pupil in a 12" scope *without* filters just looks washed out, and the magnification is too little for most filterable objects.
That is, given the choice between a filter at 4mm exit pupil (which is 76x in a 12" F/5), and a filter at 7mm exit pupil (which is 43x in the same scope), the filter at 4mm wins virtually every time.
4mm is still plenty bright for even the most aggressive of line filters, and the extra magnification often means the difference between an object being visible and invisible. Many Abell planetaries are invisible with OIII at 35mm, but are just at the edge of detectability with an OIII at 21mm.
I think Red's reply was right on target. I have had my 12.5 inch F/4.06 since about the year 2000 and together, we have been all over the southwest. Last year I spent about about 110 nights in skies that probably are classed Bortle 2-4. Some very dark skies, some not so dark. It's not that an 8mm exit pupil is always better than a 4mm exit pupil but there are certainly times when it is.
As a point of reference, I consider the Veil relatively bright. My San Diego backyard measures about 18.6mpsas directly overhead. With an O-III filter on a good night, I have seen the eastern Veil in an 80mm refractor, I see quite a bit of detail in the Veil with the 13.1 inch.
If you are over 40 and certainly over 50 an 8mm exit pupil is just too large. Your exit pupil at 50 is 6mm if you are lucky. You are effectively stopping your scope down to 9" with a large central obstruction. The outer mm or so of your pupil tends to have more aberrations than the rest of the eye. I try and keep the lowest power to 5-6mm exit pupil. A 31MM works very well at F5 or F5.5. The tiny difference in TFOV is more than offset by the proper exit pupil and slightly higher magnification.
I have told this story several times here on CN but I think it is worth repeating one more time. I bought my 25 inch F/5 about 8 years ago and sold it almost 2 years ago. Being a 100% star hopper, the relative narrow field of view, 0.66 degrees with the Paracorr and the 31mm Nagler represented something of a challenge. Dealing with the ladder and pointing the scope and all, a bit wider field I felt would make things easier. Without the Paracorr, the 41mm Panoptic would provide a 0.84 degree field of view and I figured with 25 inches to work with, even if my dark adapted pupil was 6 mm, I would still have an effective aperture of 18 inches. And so when a local astronomer who I knew advertised a 41mm Panoptic at a good price, I decided to buy it.
Well, it turned out that the added field wasn't really much help, the 31mm Nagler without the Paracorr provided 0,76 degrees so the 41mm Panoptic wasn't such a big help. But what happened was that I noticed that some objects were clearly better seen with the 8.2mm exit pupil of the 41mm even when compared to the 7mm exit pupil of the 35mm Panoptic. I just figured it was question of brightness gradient, poorly defined vague boundaries are better defined at a lower magnification.
But after a while, I got to really looking and I realized that an 8mm exit pupil was brighter to my eye than a 7mm. The skies in question are reasonably dark and most often but not always, this was with an H-Beta filter. By conventional wisdom, it just didn't make sense, I was within a year of being 70 and it was difficult for me to imagine that my dark adapted pupil was significantly greater than 7mm. I discussed it via PM with Glenn LeDrew and I ended up deciding to measure my dark adapted pupil.
I decided to do it photographically. A dark closet with a camera imaging my eye with a calibration standard held next to it. Getting good focus was a bit tricky because it had to be done manually, trial and error and every trial meant that I had to readapt. But I was able to get a good image or two. When I measured the diameter of my dark adapted pupil, it turned out to be 7.7mm-7.8mm. This was not a long dark adaptation, no more than a minute in the dark closet.
So now every time I see mention that one should consider themselves lucky if they're over 50 and have a dark adapted pupil of 6mm, I cringe. I think of all those years I figured I was just like everyone else and that I was lucky if my pupil was 7mm. If it's 7.8mm now at age 70, what was it 20 years ago when I was 50?
My take away is this: For most purposes a large pupil like mine is not an advantage, in general, one sees more at smaller exit pupils rather than larger exit pupils. Hunting down small, faint galaxies is better with a 1mm-2mm exit pupil than with a 4mm exit pupil or larger exit pupil. But there are situations, moments, when it does come in handy. This is probably enhanced by the fact that at my age, I almost certainly have cataracts which dim the image.
I believe this has been posted in this thread already but it is worth looking at.
20 to 29 years (n=66), 7.33 mm (range: 5.7 to 8.8 mm)
30 to 39 years (n=50), 6.64 mm (range: 5.3 to 8.7 mm)
40 to 49 years (n=51), 6.15 mm (range: 4.5 to 8.2 mm)
50 to 59 years (n=50), 5.77 mm (range: 4.4 to 7.2 mm)
60 to 69 years (n=30), 5.58 mm (range: 3.5 to 7.5 mm)
The important thing is not the average, we are individuals not averages. What is striking is the wide range of pupil diameters.. 3.5mm to 7.5mm, that is more than a factor of two. It is best not to make that same mistake I did and assume I was average.
If I were putting together another set of eyepieces for a 12.5 inch F/5 and I were forced to choose between a 31mm Nagler/30mm ES and a 41mm Panoptic/40mm ES, I would definitely go with the 31mm. I said that early on, it's just a much more useful eyepiece. As Red said, at F/5, the 41mm Panoptic is really a niche eyepiece, one to add on when other more commonly used exit pupils have been taken care of.