It is great the Caleb is working on adding filters to librsvg. Soon, we'll have nice support for all sorts of blurs, masks, and the like. I believe that this will lead to a significant improvement in the quality of SVG icon themes.
However, these will definitely not detract from RSVG's speed in cases where where these filters are *not* used. The way filters work is they take a base image (i.e. what you've already drawn using our lightning-fast rendering engine) and transform it according to a set of rules.
Granted, to start supporting some features (and make others work 100% correctly), RSVG will need some internal changes. But I imagine that these changes will make RSVG resemble XSVG's internally. Considering that XSVG is a fork of RSVG, I don't think that'll be *too* drastic of a change for us to re-merge.
I think that RSVG will be the full-featured renderer that you want. I think that it will be the fast renderer that you want. I do not think that replacing RSVG with KSVG is a good idea. I do not think that replacing RSVG with XSVG is a good idea.
And yes, application writers need to start using the new GdkPixbuf and GtkImage APIs that I've added in GTK 2.3. Too much scaling is happening after load-time, and as a result, our Scalable Vector Graphics aren't looking as nicely as they should. Bravo to Gnome-Games for getting this right!