I'm using C++/GLES2 for iPad only (no iPhone). The only device I own is an iPad2 and I do not plan to support iPad 1.
Can anyone help me understand realistic goals/limits on iPad 2/3/4 in terms of polygon count, etc?
There are a billion different things that can impact performance in your app. It's silly to ask "what can I expect from the performance" of an architecture, since generally people aren't going to be able to say anything more than a repeat of the spec sheet. What you're going to need to do is write whatever game you plan on writing, profile, and then fix bottlenecks.
That being said, from the small amount of iPad programming that I've done, the most expensive things to look out for are translucent (not transparent) alpha blending and fill-rate. In other words... try to avoid writing any single pixel more than once.