Hmm, very curious indeed. I did a benchmark with my 1070Ti with --mt 5 on Linux and didn't notice a hashrate decrease for 383/384 epoch threshold. Could you try benchmarking without --mt?
UPD: What OS are you using? Win 7 by any chance?
How I typed from start. Mem tweak doesnt work anymore. Now you can see its due DAG size - specified EPOCH number. Another fact is Mem tweak just works for GDDR5x memory. Your 1070Ti is just GDDR5. But its there small hashrate drop but not significant like with mem tweak switch.
Also look on that generating times for DAG which proove mem tweak is off. (8 secs with mem tweak and 12 secs without him)
20201121 13:45:23 WARN: BENCHMARK MODE (ethash)
20201121 13:45:23 WARN: EPOCH 383
20201121 13:45:23 WARN: GPU #0(000100): ASUS GeForce GTX 1080, intensity set to 20
20201121 13:45:26 GPU #0: generating DAG 3.99 GB for epoch 383 ...
20201121 13:45:38 GPU #0: DAG generated [time: 12377 ms], memory left: 3.58 GB
20201121 13:45:55 GPU #0: using kernel #2
20201121 13:45:56 Total: 21.47 MH/s
20201121 13:45:58 Total: 21.48 MH/s
20201121 13:46:40 WARN: BENCHMARK MODE (ethash)
20201121 13:46:40 WARN: EPOCH 384
20201121 13:46:40 WARN: GPU #0(000100): ASUS GeForce GTX 1080, intensity set to 20
20201121 13:46:43 GPU #0: generating DAG 4.00 GB for epoch 384 ...
20201121 13:46:55 GPU #0: DAG generated [time: 12389 ms], memory left: 3.57 GB
20201121 13:47:13 GPU #0: using kernel #5
20201121 13:47:14 Total: 20.87 MH/s
20201121 13:47:16 Total: 20.88 MH/s