Congratulations on the upgrade.
I "crossed graded" from 160 to 260, through Digital Transitions out of NY, and have had mine for about 2 weeks. I had demoed 260 in mid June against my 160 and felt the "cross-upgrade" was worth it.
I have not tried any longer exposures than 10 seconds heat here really won't allow it. The rules for long exposure are the same as I understand them as with a P45+, 1 hour exposures in outdoor temps 69 degrees or lower and it will be a while here before that occurs.
I feel that iso 50 has a bit more head room than iso 50 on the 160 (which was the same chip as the older P65+ especially with highlights. Shadows show a smoother tonality but I currently don't feel you can push them any more than with the 160.
Wifi, overall not that impressed as I thought I would be. As you point out there is a bit of a lag as the images load (2nd gen ipad). But to me the single biggest issue is that the images load very dark and Capture Pilot seems to over-ride the brightness indicator on the ipad.
There is no adjustment in the app for brightness that I can find. The retina display is not fully utilized currently by Capture Pilot (Phase One knows this and promises improvements maybe). In my tests it took 20 seconds for the image to become static (not have the wheel turning at the top). You can zoom in when the wheel is turning, but if you do this a lot it seems to lock up the ipad/back connection. For now the 100% zoom is a bit jpeg'd and I tend to zoom down a bit. You can put the ipad display and back display side by side and see that the back LCD display is much brighter. Many feel that the ipad mini is a better tool for review. I tend to disagree as the mini has even worse mirroring on the screen in any ambient light. As the images are currently displaying too dark for me, I feel that the mini would make playback review harder even using your body to block the light. Currently I use the adhoc connection. In a wifi rich environment it can take a bit longer to hook up as there are so many connections floating around.
Color shifts my opinion is still out on that. I am only using tech camera lenses so far. The SK35 might have just a bit more color shift than it did on the IQ160 on shift, center on it's the same.
I am only using Capture One on the files, as I don't like the way LR handles LCC processing. However you might try some more comparisons between LR5 and Capture One, especially in the LE shots. Capture One might be able to pull more detail from these shots. The LE shots carry a different Metadata and may require some processing LR can't do currently (like sensor plus shots). The other thing to consider is that in Capture One the defaults for noise reduction load in at 50, and I always back these off. You will definitely see more detail than with the defaults.
Iso 200 and 400 shots that I have taken so far in LE mode hold up better than the 200/400 from 160. Especially in highlights. Shadows start to show noise still.
Color, I find I definitely prefer the look of the 260 file over the 160/P65+ file when loaded into Capture One. I know this more subjective with a Digital back but I find the 260 files give an easier starting point.
I am still trying to get my head around the fact that the Dalsa chips really only record higher iso in the meta and it seems you don't really push the chip to higher levels as with a CMOS chip. This was shown very clearly in a post over on getdpi.
in a post where examples show this between iso 50 and 200 on a IQ180 by Wayne Fox. The question is have is how the newer chip handles this especially in the LE mode.
Screen display on the 260, I leave mine at 80% just like on my 160. I believe the screens are the same between the two. After looking at the wifi solution you do tend to realize just how good the IQ LCD is. In outdoor lighting where I shoot 100%, if I have the screen any dimmer than 80%, it's not really easy to read at least for my eyes.
Power off, I agree is too easy, but I had the same issue with the P45+. The 160 took more of a push to turn off. It would be a nice addition to have a button pop up on the screen to confirm power off? possible firmware upgrade?
The B&W preview option is a nice feature, as in bright outdoor lighting it's often easier to see the finer details in B&W screen mode. Has no effect on the image as I understand it, just allows you to see output as B&W image on camera LCD. Coming to older models, IQ140, 160, 180?