A lot of times that is in the manual so I'd check there. It will say how populating different slots changes their bandwidth. If it's not there then I don't know. You might have to get creative with diagnostic tools or test it with benchmarks. Perhaps someone has already posted that online so I'd do some Google searching too.
The good news is that extender cables aren't overly expensive and it's useful to have one around if you work on your own machines. So even if you use it for a while and put it in a drawer you might want it again in the future.
A bus station is where a bus stops. A train station is where a train stops. On my desk, I have a work station…