1) The in-body shake reduction helps, but only just a little. Depends on a few things really, what kind of shutter speeds were you looking for, what sort of subjects, which lenses, etc.
2) Weather sealing on my k-5 has been tested and proven. I've ran my K-5 and DA*55mm under the sink to prove the point to a friend before
3) The grip is a matter of personal opinion, you kinda have to hold it to know if its for you or not. Without the battery grip, it feels a tad too small although quite comfortable in my hands. With the battery grip, its as close to perfect as it gets. In my brother's much larger hands, it feels awkwardly tiny to him without the grip. Stick a grip on it and he won't give the camera back
So it depends really, you may or may not be better off. You could always rent one if you really want to know for sure before committing. The built-in SR works just as well as my friend's VR equipped lenses, so if that's good enough for you, then go for it. Once again though, i must stress that the effectiveness of the SR has its limitations and there are a ton of variables that can come into play in regards to determining its effectiveness. I recall someone on the forum doing an in-depth test of the SR system, but I don't remember where i saw it and on which camera.
As for birding lenses, i'm not much for birding, but do own an old 300mm f/4 manual focus lens that works brilliantly for soccer.