The real thoughts behing tenacity is : right now, is it really necessary to have this extra mitigation ?
Once you'll have the 340 set, even O4S should hit less harder.

The next hard content waited is the Super Savage.
So, in others words... the only reason to have a full 340 tenacity set would be that Super Savage will hit so hard that you definitly need it to survive.
Otherwise, it'll be better to increase the values of the dps.