Re Panasonic lens on Olympus...There are a couple of things. One is what Wapiti mentions above, that the Panasonic lens on an Olympus body is more prone to flare, can exhibit the purple blob issue and is more prone to chromatic aberrations. The other is that Olympus and Panasonic seem to be diverging a bit on things like stabilization and the Olympus E-M1 ii offers a feature called pro-capture that will not work with Panasonic lenses. That feature may be of no importance to you. I intend to try the Panasonic 8-18 when it comes out knowing that it won't work for pro-capture.
I have no idea how significant distortion of 7.2% vs 5.2% may be, as viewed in an image, so those numbers don't mean a lot to me. I also simply expect there to be some distortion in such a wide lens. If the one article I cited earlier is correct that the Oly is actually much closer to a 6mm at the wide end, then that, too, would likely explain greater distortion at its widest point than the narrower Panasonic. As far as I know, lenses are no longer designed to correct for distortion themselves. Manufacturers rely on in-camera software. I shoot RAW so I don't get the benefit of that, although you can correct for distortion in photoshop and lightroom.
When we are talking about distortion with these lenses, we are talking primarily about the ability of a lens to keep a straight line looking straight. When I shoot underwater, there are few straight lines and one of my favorite underwater lenses is the 8mm fisheye, for which straight lines simply don't exist. So, I don't care a lot about distortion as much as vignetting and sharpness (to a point). Of course, if what you shoot requires minimal distortion, or if it simply matters to you for other reasons, that is fine. Generally, I doubt anyone could identify which lens was used on any given image.
"The Olympus 7-14mm ƒ/2.8 Pro is currently the widest rectilinear lens available for the Micro Four Thirds system with a constant ƒ/2.8 aperture, so there really isn't a direct competitor at the time of this review. The most similar alternative, however, would be the equally wide Panasonic 7-14mm ƒ/4. Though not as bright with its ƒ/4 aperture, the lens offers the same range of focal lengths. Sharpness is very good, though its CA, vignetting and distortion characteristics aren't as nice as the Olympus'."
"When keeping the field curvature in mind and act accordingly, you can certainly achieve very good results. The center quality is breathtaking whereas the outer image region has be handled with care when shooting below 10mm at least (pull the focus a bit). Image distortions aren't relevant from a user perspective."
“The distortions are very well controlled and very discreet on this objective: a very good point! We are, however, somewhat disappointed by the optical quality it produces, and more particularly by its lack of homogeneity between the center and the edges of the images. By comparison, in the world of the reflex 24 x 36 mm, it is much better. However, the center and the sides of the images are well pricked and are full of fine details. We also appreciate the very good control of distortions.”
It seems that the lens does suffer from "field curvature", but keep in mind that this is may be due in part to the actual angle of view being quite wide, and in any event hard to evaluate when you are not shooting flat objects. I rarely have underwater photos where the center, the subject and the edges are the same distance. Also, I always try to remember there are substantial sample to sample variations in lenses, particularly zoom lenses, so any single review may not reflect what you will experience.
In the end, I think both lenses offer similar performance with the Panasonic doing so for less money, so that may be the better "buy" long as the limitations of using a Panasonic lens on an Olympus body are acceptable.
If you are undecided about the lens and have time, you might want to wait a bit and see how the Panasonic 8-18 compares and what port options are available for it.
I think the closer focusing Oly 7-14 can be beneficial if you are using the lens to shoot smaller items or for CFWA. I can't really quantify how valuable that benefit is or isn't. It depends a lot on how you shoot. I only shoot raw underwater so I don't know how the lenses compare in jpegs. In raw, I get cleaner images from the Oly lens on and Oly body and unless there were some other compelling reason to get/use the Panasonic, I would get the Olympus.
About the 12-40...I really enjoy that lens. I suppose it would not appeal to a lot of people because it can't do macro and isn't wide enough, so it doesn't excel at either end of what many of us like to shoot. On the other hand, it provides about a 1:3 ratio which is fine for small fish and other critters and at 12mm it does a nice job on reef scenes, corals and so on. I know it is popular with people who do insect pictures as well because it focuses close enough at 40mm that you can pretty much fill the frame with something the size of a butterfly or even a bumble bee.
I do some macro, but for the most part I like shots of things that shows something of their environment or some background for color. IN those instances, I think the 12-40 is more useful than my 60 macro. IN a pinch, it can do some cfwa, but isn't as wide as one might want. Still it is one of my favorite lenses to use underwater.
I am pleased to hear that. I hope that holds. I have heard that new scanners are available or about to be, that would permit laptops and liquids to be effectively scanned and end the liquids ban and perhaps remove the threat of a laptop ban. I don't know if that is true, or if they will be implemented, but maybe.