Yeah I have a 5500ps with 6C, you still wont hit those brightest RGB blues though, even with a Hexachrome 8C, Though better then a 4C.
If your working in RGB what you could do is download the colour profiles for your printer and setup illustrator to proof on screen. All the profiles should be on the printer manufactures site, should have profiles for all the different paper stocks, inks, dyes.. ect. Once you've downloaded your profiles and added them to the comp. go to View > Proof Setup > Custom and pick the profile which matches what your doing. Now your working in RGB but your getting a soft proof of the printers gamut displayed so your not totally out of the ball park. Once you print though your sending an RGB file to the printer and your letting the rip decide how to convert the colours.
The better way of setting stuff up is to setup your colour management and give your file a profile. You would be working in CMYK again, but you would be working in the CMYK profile of your 6C which is larger then the default Ilustrator settings. And then you set up all your other programs to match your workflow... and on .. and on... and on... this could go on forever so I will stop. Im no colour expert anyways, im happy as piss just working in SWOP. Maybe a colour god on here can go though the whole colour thing better then I can.
Note: Colour proofing is useless without a good monitor.. good monitor that is calibrated.. a good monitor that is calibrated regularly.. and for digital, your printers calibrated regularly too lol