I believe they said that Tier 8 delves will give equivalent to Mythic+5 gear, and that's the cap for delves. So, pretty decent really. I think that's right around Heroic raid gear? Not sure, but I seem to remember hearing that at some point.
The limit on best gear from delves per week will be the number of treasure keys you acquire per week, which we don't know yet how they are acquired or how many you can get per week.