We are using an Onyx RIP on our Canon iPF9000 (60" wide printer) and have tested it extensivly against uprez programs (IE: Genuine Fractals) Old School techniques (IE: 10% upres incrementally using PS) Just reducing PPI to get the desired sizes, We also compared it to the mechanical uprez of a LightJet we had in the shop.
For us the scaling is up to the same par as the mechanical LightJet uprez which ended up being better than all the other techniques.
The colors printing from BetaRGB color space and letting the RIP do its work seems to have better color handling as well. There just seems to be a better pop to the prints on the bright white matte I print on.
I am not sure what the ColorBurstRIP has in the ability to help with colors, but dosn't look too darned expensive and being it is from Epson, (which I have had some prints printed at a local pro shop on an Epson with a ColorBurstRIP and they looked up to my normal quality standards (finiky :~) These were 40" x 60" prints from 12MP shots and held up very well.
I am a big fan of RIP's so far...I am not thouroughly versed in every RIP out there, but so far they have proved merit to my persnickety tastes.
Hope that helps.
Roman