I can confirm that the difference between higher than 50 Legendary mods seems to be pretty puny. It's actually a relief, getting these Legendary mods is a pain in the ass so at least it's not a priority.
For most relics, the difference seems to be about +10% of the level 50 value. A perfect 61 vitality relic gives +54% health instead of 50% at level 50. A 61 protection relic gives +33% recharge rate and capacity instead of +30%. I think the exception is unique relics. There Sheriff's Badge changes an insignificant amount, for example.