Will Wifi throughput be affected by RSSI (attenuation) in my setup?

Hi;
Yes, definitely, signal strength will effect your throughput.
Because each receiver has minimum input sensitivity (which defines how low power signal can be captured and decoded).
Must satisfy signal levels defined by ieee 802.11 spec. Have a look table in this page. However your device receiver may be better than given figures. That means better tput performance.

Let say you have -50 dBm signal, receiver can sense 1024QAM signal which is good throughput.
When you increase attenuation, signal level decreases and (let say so that signal level is -62 now), receiver couldn't sense 1024QAM signal any more then modulation rate is adaptevely decreases so that both radios can communicate correctly, which will be 64QAM. This is 16 times lower throughput.

Given figures just to make it clear. Actual tput depend many parameters (antenna, noise coming from nearby circuits, signal strength, bandwidth, transmitter quality etc etc)

Hope help.
Good luck!

PS: Given setup is well known setup for RvR (rate vs range) tests, it is used to simulate AP-STA physical distance (which will happen in real world) in a controlled test environment.