100+ FPS is consistent with what we observe for ECO mode ON, so it appears to be functioning as intended.
Note that it only pauses the rendering-heavy passes. Other GPU processing may still be going on. The biggest GPU consumer in that case is usually the UI. If you are on a single-GPU system, some GPU load is expected.
In the future, to free up the GPU completely, we may implement offloading of the remaining tasks to the integrated graphics or the CPU.
If you would like such a feature sooner, it would be helpful if you could describe your use case and its importance, so that we can better prioritize.