I'm wondering if these refill inks might be worth a look. Sudhaus cartridges are well reviewed on druckerchannel.de, so I'm guessing the refill ink should be cool also. Most people in here seem to appreciate ink by Image Specialists or Octoink etc. But Sudhaus is a major German commercial manufacturer so I'm curious about the quality. 
Check it out over here:
http://www.patronenhandel.de/shop/en/Redsetter/IP-Serie/iP-4500/mit-Refill-Set
Check it out over here:
http://www.patronenhandel.de/shop/en/Redsetter/IP-Serie/iP-4500/mit-Refill-Set