I don't think enabling Open MP will bring much to net render if you are already maxing out your core usage.
My understanding is that A:M currently only uses GPU processing SSAO and for certain post effect filters, so general rendering is not sped up by the GPU. I can't find where those filters are selected at the moment. But if you are using SSAO you may want to enable it. When "Use GPU" is off or if the card does not have the features necessary for GPU SSAO, A:M uses a slower CPU version.
However, I would encourage anyone shopping for a new graphics card to get one that supports real-time ray tracing under DirectX 12, in case that can be somehow incorporated in a future A:M.
Graphics cards prices are currently insane due to bitcoin miners and pandemic production problems, so waiting a bit before spending money might be wise.