Are these "exceptional preceptual rendering characterisics" simlpy better, or can be tweaked to a larger extent?
Some profiling apps simply have better perceptual rendering but also also for tweaking of the behavior. For fine art printing, exceptional perceptual rendering is super important. It can hit a sweet spot that allows for excellent edge gamut saturation without losing detail along with excellent gray balance, shadow detail and black approach handling. IMO, MP and i1P are clear winners here.
I use Epson 9900 and 4800, with latest drivers, and Mac OS X 10.6.6.Monitor: EIZO CG221.I use the drivers normally , but i had some very interesting results with X-photo in demo mode so I would ask for the opinion someone's who has already worked with either the one or the other rip in order to be more certain about my conclusions.
I've had literally hundreds of ColorBurst and Imageprint installs in demanding fine art environments over the last decade and am super familiar with the quality differences. As Andrew said, IP offers ink and gamut conservativeness while the driver and CB can each equally achieve a greater gamut and DMax. Again, the role of an excellent ICC profile plays a more important than the RIP or driver itself. If you're working with eccentric materials like silks or handmade Japanese papers, CB's custom ink limiting and linearization lets you customize to the material better than the driver (if you're up to the task of mastering the process). I'd go with CB over IP for it's ink limiting, linearization and gamut capabilities. If you're just working with common RC, fiber base and cotton rag materials the driver does the same job with less fuss and greater simplicity.
7+ years ago RIPs had distinct quality improvements over printer drivers. Things have changed to the point where you'd be crazy to use a RIP unless you absolutely need the features that they offer that drivers do not (long print support, nesting, tiling, layout, customized calibration for eccentric media, etc). Whatever you do don't buy the hype that "you have to have a RIP to do great quality fine art printing." For top notch fine art printing I'd suggest focusing on the media, profiles, print sharpening technique, localized contrast technique, noise/grain enhancements and other fine details worth mastering.
It sounds like you've already made some conclusions. Why don't you tell us more about the results with CB that you found "interesting"? Give us all the details and let's discuss them. Be thorough and precise and give us the specifics. What papers, what profiles, what type of imagery, what are you looking for...