On a stock S9 I can confirm that the LPM firmware either modifies or replaces the bmminer file, I would assume it would replace a modified bmminer as well. The LPM bitmain bmminer does not work with slushpool.
I added Taserz bmminer after the update to LPM and it does work with slushpool. This modified bmminer seems to run faster with LPM checked, maybe someone else can confirm this, I don't have time to check on multiple machines at this time. bmminer880 650 freq 13.950 th/s. With LPM unchecked I see readings closer to 13.825 th/s. I don't have access to power readings at the moment unfortunately.
All my machines are currently away being hosted rats....
I have a feeling if you do firmware upgrade. Then replace firmware the LPM toggle won't do anything. I checked bitmains repo and they did not release the source for the new firmware.... For a 5% power saving I am seeing on the forums I don't think it is worth switching over to it. Granted if LPM toggle does work which meh idk and we can still overclock and tweak the settings it is def worth it. But I am at a standstill on figuring that out hopefully Slush's team can before me but with a lack of resources it's a b&@ch.