Well, yes and no. As far as I'm concerned, 2-inch eyepieces and serious portability is a contradiction in terms. There are individual 2-inch eyepieces that weigh as much as an entire 60-mm spotting scope -- and don't bulk much less, either.
Moreover, field curvature limits the useful field of view of any small, light refractor. If I notice dramatic field curvature in my 80-mm f/4.4 scope using a 24-mm Panoptic, just imagine how bad it would be with a 40-mm Panoptic! To say nothing of the 9-mm exit pupil.
For my money, a refractor in the f/5 range with a 1.25-inch focuser is the best possible compromise between wide field and portability.
Incidentally, if the OP wants something really tiny and cute, consider the Stellarvue 50-mm f/4 finder. It fits easily in a coat pocket, yet splits Castor handily with a 3-mm eyepiece. Not a lot of light-gathering capability, though.
For me, this gets to the heart of the matter. Is the primary reason to choose a 70mm or 80mm scope it's compact, lightweight nature or is because of the unique optical capabilities it offers? Initially, the question seemed to be about the difference in capability between the a 70mm and an 80mm, what would be seen. It wasn't until the second page that Bowlerhat mentioned traveling by train. For me, portability is not the primary concern, it's what the scope can show me.
In terms of field curvature, I do have quite a bit of experience and understanding, I've done a lot of calculations. ST-80's with 2 inch focusers have become quite popular, I have one though it's loaned out right now. I also have an 80mm F/6 and an 80mm F/7, both ED/apo's. I don't have to imagine the field curvature visible with an 80mm F/5 with the 41mm Panoptic, it's a combination I often use to get the widest, brightest view, that's 6.6 degrees at about 10x with a 8.2mm exit pupil. I used to be afraid of exit pupils greater than 7mm and probably most people should be but that was before I discovered that my dark adapted pupil is closer to 8mm than to 7mm. There's field curvature visible, quite a bit to be sure but the views across the field are cleaner than my 10x50 Ultraview's.
I generally prefer the ST-80 with the 31mm Nagler which offers a 6.0 degree field at 13x with a 6.2 mm exit pupil. With well corrected eyepieces like the Panoptics and the Naglers, the field curvature is visible but I do not find it particularly bothersome. In part, this might be because most of my wide field, low power viewing is done with a 4 inch F/5.4 TV apo that is corrected for field curvature, it'll do a 4.9 degree TFoV with stars that are sharp-sharp right to the edge. When I am using an 80mm, I am looking for a wider field than that and I am willing to accept that field curvature as the price to pay.
My most recent good-high quality apo/ED was the AT-72ED, it has a 430mm focal length so it offers a wide field but somewhat narrow field, somewhat dimmer view than the ST-80 with the 2 inch focuser. It is a rather heavy scope for a 72mm, not much less than either the 80mm F/6 or 80mm F/7. It is a nice scope for someone who doesn't have multiple choices at 80mm but for me, it gives up too much at the low mags and the high mags. It is not enough smaller that a lighter mount can be used.
We all have different situations and different priorities, we all have different alternatives. I don't travel much by plane and when I do, I take binoculars. I do travel to the Boston area to assist colleague with his research. I recent arranged for him to purchase a Zhumell 8 inch Dob so I have something to observe with while I am there. Most of my dark sky observing is done from our second home in this high desert with the remainder being done traveling about the southwest in our motor home.
I do like 80mm scopes, my primary interest is the optical capabilities they bring to the table with portability being secondary.
As far as the StellarVue SV-50 F/4 finder. I have one, I like it and like you, I can split Castor with a 3.5 mm. The clear aperture of the diagonal does not allow the use of the 27mm Panoptic without vignetting but it still provides a 6.8 degree TFoV at 8.3x with the 24mm TV Widefleld. With it's 200mm focal length, there's definitely quite a bit of field curvature but the views are still enjoyable. The 16mm T5 Nagler provides a 6.3 degree TFoV at 12.5x with a 4mm exit pupil. That's a surprisingly good view and I often use a O-III or UHC filter with that combo. Even though it's primary purpose is as a finder, it is a very capable RFT. CN member "Northofsixty" has an SV-50, also called the Little Rascal, that he uses as his primary scope. He's lives in the Yukon.
In terms of small RFT's, Sheldon Farowski sells an air spaced 70mm F/4.5 objective, it's new-old stock, made in Japan, $26. I modified a Orion 70mm finder that had poor optics with one. I split Porrima with it last month, it provides a 4.9 degree TFoV, weighs under 2 pounds with a diagonal.
Anyway, enough rambling. I like small scopes in this 50mm-80mm and even 100mm range and do quite a bit of observing with them. For me, my primary interest is not their compact size, it's what they show me at the eyepiece.