Hey Rusty - I don't have any knowledge either, but it seems worth a try for the kind of sailing we'll be doing for the next couple of seasons (Bay and close-in Gulf).
I'll answer each of your points with my completely uninformed assumptions...
I concur with most of your points, but would add the following:
1. iPad is hard to read in sunlight
-I have the iPad 2. It's pretty good. And I also have a bimini, so that might help.
*I've been underwhelmed with iPad performance in sunlight just for general use, let alone a fixed mounting at the helm with the sun over your shoulder, and with other issues to deal with.
6. iPad has limited connectivity (is there even a way to display radar or sonar?)
-From what I read, if the signal is Seatalk/NMEA 0183, the iMux will push it to the iPad. So, if it's digital radar/sonar that is Seatalk compatible - yes, I think so.
*I know even less about boat networking, but I've read that 0183 is similar to PC serial bus = very slow. I doubt either has the bandwidth to transfer moving images, like radar or sonar. Wifi would be more likely, but then the radar or sonar needs to communicate by Wifi, and I haven't heard of any that do.
7. Most of the new chartplotters have sonar circuitry built in, which iPad does not.
-True. I'm not worried about sonar for now. But again, if it's Seatalk it seems it should work.
*Assuming that bandwidth wasn't an issue (see previous), the iPad would only display the image, which means you'd still need some sort of separate sonar unit to generate and process the sonar data, which many chartplotters do themselves.
There are dozens of threads, here and elsewhere, about using an iPad as a chartplotter. My understanding is that is exactly what the high dollar racing teams are doing (though my guess is that they have an old fashioned chartplotter on board too).