I think I see what you are thinking. Unless you have upgrades your case by adding threaded inserts to retain the inner primary, you are limited to the torque you can apply to create the sealing effect from the copper washers.
There was never a critical structural need to support the primary inner at the case, however the increasing subject of creating a seal point to help keep oil out of the dry primary cavity creates a need.
I think it is safe to say, torque won't do it. If not copper washers, perhaps rubber impregnated sealing washers like the petcock type might be in order. Of course the need for protection in regards to the tabbed washer is realized and should not be discarded.
As far as sealing the stock threads as is sits, an RTV of almost any type should do or maybe even Permatex #2.
Anyhow, threaded inserts in this application are awesome. Not to get away from your topic, but anyone who may have their cases split should seriously consider doing this simple and highly effective modification. Drill it, tap it, set em in with JB. You can get them at any hardware store for 50 cents a piece. I think it is safe to say that you can add 50% more torque to the fasteners, if not doubled.