I testet it on a March 2018 Batch - it is a 13,5 TH S9
No, I selectet 631MHz, as I know, that this is 13,5 TH in total. Stock speed??? There is no "stock speed", there is usually "autotune" and the controller board is checking, which avarage of frequency each hashing board (chain) with the 63 ASICS is able to perform to reach maximum 13,5 TH.
If you have read the bmminer source code, you will have recognized, that the firmware is limiting the possible hashingrate with a maximum of 13,5 TH. So any increase of defined parameter which would lead to a higher hashing rate is not possible.