has anyone tried putting a Sako style extractor on a 788 like the one offered for the Rem 700 by midway?

On first blush I'd say it wouldn't work, the 788 has rear locking lugs where the 700 locks up up front. One of the resident gunsmiths will be able to tell you for sure

As long as the thickness of the front of the bolt is the same, you should be able to. The lugs don't even come inot play. I have never installed one on a 788, but have done a few 700's and 722's. Maybe someone else will know for sure, but from what I see, you should be able to.

Maybe you should talk to a gunsmith about it. I haven't owned a 788 in over twenty years, don't remember how the bolt face is configured but I know the 700 bolts fairly well. There are many gunsmiths that say this is a bad idea on the 700. One of the features that make the 700 such a strong action is the "ring of steel" at the bolt face. In the event of a case rupture, the ring is designed to contain the pressure. When you cut the ring to install the Sako style extractor, you have compromised this safety feature. This issue is probably best described as a "personal choice" and I don't know if it would apply to the 788 so this is just food for thought.