The HPZ3100 currently has a red gamut performance through the windows / mac driver which is slightly less than the machine is capable of producing.
Top-line RIPs - Onyx, Wasatch etc can drive the printer natively and achieve the same full red gamut as the Canon ipf printers.
The current firmware for the Z3100 addresses the red issue to a great extent and it is now much improved.
Anyone using the older firmware is encouraged to upgrade.
HP are aware of the issue and are working on a final firmware upgrade which will bring the reds to the same level as that achieved by the big RIP companies.
If you are looking at RIPs be aware that most do not drive RGB printers properly as they do not provide a true 700% ink limit despite showing CMYKRGB patches in the profiling software.
This may be interesting:
http://www.onyxgfx.com/news.pl?cmd=view&nid=49Onyx have a new low-cost RIP which has the full-on colour engine of the main product.
It may be an answer to running the Z3100 at full gamut without spending another fortune!