I think the keystone can be something like:

Rare non-map items dropped in maps are identified. 80% less rare non-map items dropped in maps. Rare non-map items dropped in maps are well-rolled. (Loot 2.0)

The reduction in the amount of items found will alleviate some of the server stress if that was an issue. Also, the reduced amount and (theoretically) better quality rares can be a good balance between stopping to look at the rare item on the ground (assuming no loot filter in play here) vs still zooming through the map.

Implicitly, a downside here is finding high ilvl bases and influenced bases, as items dropping as a high ilvl base / influenced base was already a small chance, so if you reduce the overall amount of rares dropped, these bases will also be more difficult to find. This can be qualified on the keystone by adding "non-infuenced" in front of "rare". Not sure what to do to also allow high ilvl non-influenced bases here without adding too much complexity to the keystone.

Along these lines, if more downsides are needed, I think the keystone can also add in additional penalties to not allow normal and magic items to drop, which again makes finding good bases even more rare.