Lensrentals posted an article,
There Is No Free Lunch, Episode 763: Lens Adapters that says that "...putting a great lens on your camera via an adapter might still be better than an average native-mount lens. On the other hand, that great lens certainly wouldn’t be as good as it would be on its native-mount camera." Something I think we've always suspected... But it's interesting to see some numbers behind it. Seems it really isn't such a great idea (if it can be avoided) but especially when combining them.