Basically everything you said is wrong.
1/ Openxcom always uses software rendering... with this option enabled, it does everything it does also without it and at the end forwards the software generated image to OpenGL for hardware rendering (and optional further processing via display filters).
You will (mostly) not see any performance improvement here, it will (mostly) only get slower or stay the same.
2/ Also, you shouldn't use this option as a user at all, it's an internal option and the game can decide to overwrite your setting based on other options as it sees fit. Or it can crash or just be ignored, depending on circumstances.
The correct way of turning on raw OpenGL rendering (without any display filters), is to use 'Options > Video > Display filter' and select option 'Raw*' from the drop down box.