The total modifier to damage received cannot go below -50%. So, that Nano’ed Sojourn would still only have 50% damage reduction.
The difference would only be tangible if she also had a positive modifier to damage received, which currently can only come from Orb of Discord. So, without this hypothetical passive, a Nano’ed Discorded ulting Sojourn would have a total damage received modifier of -25% (i.e. 25% damage resistance). With the hypothetical passive, the total damage received modifier adds up to -55%, but that’s below the floor, so she’d get -50% (i.e. 50% damage resistance).
All that said, I don’t think this passive makes sense. With so much less CC and damage mitigation in the game, there are many more situations where the only viable defense is to focus the ulting hero down with damage. If that becomes 30% harder for DPS ults, that seems like a big problem. The interest in playing support seems quite low already.