vertices always need to be split to achieve hard edges. not matter what. the GPU/direct3d/opengl can't do this in a different way. so blender's edge split modifier is perfectly fine! actually it's perfect because it saves me the work of converting something like smoothing groups to split vertices.

i am not sure if split vertices make much performance difference. there are caching mechanisms which possibly could catch this and cause calculations to be done only once for vertices at the exact same position. probably split vertices are a bit slower with animations though since gamestudio does animation in software and probably doesn't have such a caching mechanism.