Remington (and other ammo manufacturers) typically don't publish their chamber pressures because they may fluctuate -- there are obvious legal implications. Unlike components for re-loaders, factory components may vary from lot to lot. The ammo manufacturer will adjust his recipe, from lot to lot, to maintain advertised velocity while remaining under the maximum allowed pressure. Your assumption that the Rem. Managed Recoil loads have a lower pressure is most likely correct, except it might not be what you define as significantly lower.
From your post, it seems you are looking for a lower pressure load because you question your old SxS's ability to operate safely at the maximum allowable pressure. If this is the case, IMHO, seeking a lower pressure load is a risky gamble. Have the gun checked by a competent gunsmith, you'll sleep easier.