Adding RAM will not significantly increase the speed of Fax jobs. Please note that in the tests we ran it seems to have run "slightly" faster with 16 MB. However, when you take into consideration switched phone lines and retrys to the printer the time differences noted are insignificant.
Here are the results of the benchmark test.
Test 1 - One page document
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From Start to dial tone From Dial Tone to EOJ
7 MB 1 Minute 15 seconds 39 seconds
16 MB 1 Minute 5 seconds 38 seconds
Test 2 - Five page document
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From Start to dial tone From Dial Tone to EOJ
7 MB 3 Minute 19 seconds 2 Min 46 seconds
16 MB 3 Minute 15 seconds 2 Min 46 seconds