Have you tried changing the microphone's sample rate outside FCP? If not, try closing FCP completely and open the Audio Midi Setup, found in the Applications/Utilities folder. What you see in there will depend on the devices connected, but usually you can set the sample rate for the device through there. That setting may very well overpower the setting in FCP. Then, you may be able to change in FCP, or maybe FCP will be "stuck" in 48kHz.
I've never worked with that particular device, but this would be my first guess as I've seen many similar situations with other devices. The mic may have come with its own control software also, where the setting could be changed. The manual doesn't mention this at all, so I doubt it.
There is no way at all your computer configuration is limiting your sample rate. That's pure bullocks.