Grouping will help your draw calls, yeah. If you’re using lightmaps, there’s kind of a trade off where the more brushes you group the higher you need the lightmap resolution to be for that group in UE4 to look good. The AutoLightmapUEr plugin can make that easier.
Occlusion wise, that sounds like a weird separate issue, not to do with HammUEr. Might have to read up a bunch on UE4’s occlusion systems; there are things you can tweak.