I have to say I'm not particularily impressed with the IR library. At least not for orchestral music. I haven't got a chance to try the bundled surround IRs though - maybe they are better. AFAIK, no multipositional placements apart from the surround IRs. I mostly use impulses that I've downloaded from net. I'd like to try using multipositional placements some time but right now I can't since Kontakt2 doesn't save aux send levels properly (due to a K2 bug).
I very rarely mess with the convolution in K2. The included IR's (for reverb purposes) are not all that great. Keep in mind too, that the IR loaded in an instance only applies to instruments in that instance. So if you are using 3 instances of Kontakt, each one would have to have the convolution engine enabled in the FX output, adding additional CPU overhead.
My advice is to pick up Pristine Space, Convo Boy, or SIR and download some nice IR's from the net (like the Samplicity set.)
I'm glad to see I'm not alone. So far I haven't found any K2 impulses that I feel good enough to work for me. By draging in other impulses includeing some of the ones from noise vault i get much better results.