Clueless? Hardly.
An application must be specifically written to take advantage of multi-GPU setups. As as the time of the post (4+ months back...), FCP 10.1 was the only program that could take advantage of the dual GPUs. Hence the conclusion -- adding more GPUs will likely not do anything, as not even 2 are supported.
Additionally, the FCP link doesn't answer either of the OP's questions. It neither tells whether you can install (or expect to use) additional GPUs, nor whether FCP will take advantage of additional GPUs, even if they can be installed and linked properly.
You responded "Doubt it. Getting support for dual GPUs is going to be hard enough." FCPX already utilized dual GPUs when you claimed that getting support for that would be hard enough. That link was for you, not the OP.
BTW Resolve utilizes multiple GPUs (more than 2) and had been doing so for months prior to FCPX and Motion added support.