Also remember that C2R is planning on offering the Sentinel for use with the cartridge. I'm quite sure they would have liked to get good performance out of that combo. But as Alex and Joe said, the cartridge/cannister design is difficult and has it's limitations. I was told the same by Pete Ready who experimented with solid state (cartridge like, but pre-Micropore) scrubbers for both annular axial MK15.5 and early axial PRISM years ago.
I also remember a CO2 spike in one of the tests posted a DRE. I questioned it as it looked like a brief, early breakthrough, but was assured that it was due to a mistake made during testing. That's pretty much how I envision the scrubber being overbreathed during a strenious situation, with some resting getting the scrubber to fully absorb again, as per Alex's suggestion. But taking 5 for an OC break so the scrubber can relax is less than an ideal option, more so at 300 fsw. Time to add a "scrubber relieve tank" to you gear, or ask for free DR snorkel?
Apologies for the sarcasm, but for a rebreather aimed at deep technical diving that seems rather ridiculous. 2 1/2 years ago I wrote
"Personally I find it a curious combiantion as the Hammerhead screams 'technical diving' with its trimix deco capabilities and myriad of adjustment choices. The Micropore scrubber on the other hand shouts 'recreational' just as loud."
In tests performed here the Optima was certainly very disappointing. Not just the scrubber's duration, but also the breathing resistance. The short loop, the advantage of the cartridge and o-t-s counterlungs, all highly touted by the manufacturer(s), yet the unit doesn't come in anywhere near the top.
IMHO, if the price and availabilty or the scrubber medium aren't good arguments for other units, the lack of performance certainly is. To paraphrase the Australian poet and aesthete McHatten from his OzTek article, Dive Rite is great and I love their products, but the Optima just isn't Dive Rite.
IMHO, Lamar and Kevin can do better.