" The ONT is compliant with ITU-T G.984 supporting a 1 Gb/s upstream and 1 Gb/s downstream line rate."
Here is the datasheet on the ONT Cox uses. Here is Cox's page on it, just make sure your area code is set to somewhere that has gigablast to see it. I don't see any 1Gb SFPs' in your list.
Also, I think you might get push back at the installation. Does the Gigablast ToS say anything about CPE's? I found this, but didn't see anything specific. I would think since ITU-T G.984 is capable of 2.5Gb/1.5Gb there would be concern on equipment modification. Just like they wouldn't want anyone flashing the ONT with different firmware. You might be able to get special permission, but it's a stretch IMO. Hope you do get permission though, would love to see that install. I am a wire management nut.