From what I understand, yes. Shadowplay basically doesn't do anything besides copying the framebuffer (i.e. game) to the NVENC unit. OBS does a lot, and to do such it requires OBS to composite your sources/scenes on the GPU.
How much GPU does OBS use? Depends on how complex your filters, sources and scenes are. If it's the same as it would have to be with Shadowplay, then not very much.