As a SG (or a Troupe member with a mind for "game balance"), the first thing I try to do is take things to their logical extreme. At what point does the new rule break (or break something else), if at all, and what is that point, and is that acceptable in this Saga?
If you're talking about T:Container as separate from D:Seal, then no way. This would circumvent too many T:Group effects. A "(minor) breakthrough" should be a small improvement or a different but "equivalent" angle that allows one to do something different - not knock magnitudes off spells across the board. (If used solely as a conjunct to D:Seal, then possibly, yes.)
I had an immediate kneejerk against +1 for Duration:Seal, but I was starting to think - how could it be abused?
Well, one would be if the Duration could be attached to something other than the "container" or "contents" - so that has to be made clear.
Now, if the "contents" of the seal cannot be accessed (which would break the seal), then D:Seal seems only good for:
- Preserving the contents (food, texts, prisoners, etc)
- Experimenting on the contents (warping, torturing, random experiments)
- Threatening the contents (if opened, the contents lose X effect which keeps them preserved/alive/useful/valuable*
(* Think of a magical "cryptex", sim to the box in The daVinci Code - if opened, the vinegar/spell comes out and all is lost*.)
( Does this invite a password/action/trigger to bypass a Seal?" or does that want to be a different thing?)
(And would that be it?)
If so, while all those are nifty, I'm not sure "game balance" calls for a +2 or not.
However, one aspect of a Seal (or an additional seal, parallel to others - if that's allowed, more than one "seal" per "entry"?) could prevent that seal from being broken - or attempt to. Ward vs. corpus, whatever. This would approximate a "Permanent" effect much closer than any Duration:Ring arguments - and so we lean back to +2 (or even +3?).
Also, note that we already established that the concents can be accessed - one could Seal a room, and that bypassing the Seal w/ teleportation would not break the seal. So a room where one accesses the contents w/out removing them (a lab, for instance) could be "sealed" with an effect and never need to be "opened". Part* of a Covenant's Arcane Library could benefit from any number of such "permanent" effects, and from preservation/protection effects while not being used, if the contents also can be teleported in/out, begins to make this Duration far more powerful than at first glance.
(* Lab Texts want to be taken back to a Lab to aid in inventing spells/etc, but Arts/Summae can all be studied in situ., etc.)