Of note is Bowhaus' "True Black and White" RIP software. For "no-compromise black and white work" it's absolutely fantastic and far superior to QTR.http://www.trueblackandwhite.com/
Looks interesting, too bad it's Mac-only and doesn't support the x300's yet (according to the documentation on that page, at least).
I find the Canon's monochrome mode to be good, with better linearity and DMax than printing with RGB profiles. I use QuadtoneRIP and my Eye1Pro to create profiles for the monochrome mode. The toning controls make it pretty easy to get the warm-tone prints I like for most of my B/W work, and from what I've seen the toning controls don't have any effect on density/linearity, so I can use a single QTR profile and vary the toning if I want (understanding that the soft-proof only shows the toning of the original profile settings, of course).
If you prefer a dead-neutral print and you're using 3rd party papers, you might be better off with a custom profile for the RGB printing pipeline, because using the simple x/y-axis controls in monochrome mode it might be difficult to get a truly neutral result on papers that don't quite match up with the Canon media types. It might work well with some papers, but not so well with others. For instance on Photo Rag Baryta, the "Neutral" setting in the monochrome driver produces prints with the 'a' channel in the -.2 to -.4 range, with the 'b' channel in the 1.5-2.5 range. The result doesn't look particularly neutral. I could probably improve on that with some iterative tweaking, but since I usually prefer warmer tones it's not something I have pursued. If I want a neutral print I'll just use my RGB profile.