I performed some tests with server-side timers to keep the time (regenerating from 0 to 100 mana, with 120 meditation/focus):
Mana Regen 18
Mana Regen 18 - 263ms per tick - 26,300ms theoretical regen
Mana regen took 32,096ms
Mana regen took 32,007ms
Mana regen took 32,099ms
Mana Regen 17
Mana Regen 17 - 270ms per tick - 27,000ms theoretical regen
Mana regen took 32,114ms
Mana regen took 32,114ms
Mana regen took 32,123ms
Mana Regen 16
Mana Regen 16 - 278ms per tick - 27,800ms theoretical regen
Mana regen took 32,114ms
Mana regen took 32,164ms
Mana regen took 32,159ms
Mana Regen 15
Mana Regen 15 - 286ms per tick - 28,600ms theoretical regen
Mana regen took 32,108ms
Mana regen took 32,202ms
Mana regen took 32,160ms
Mana Regen 14
Mana Regen 14 - 294ms per tick - 29,400ms theoretical regen
Mana regen took 32,171ms
Mana regen took 32,767ms
Mana regen took 32,169ms
Mana Regen 13
Mana Regen 13 - 303ms per tick - 30,300ms theoretical regen
Mana regen took 32,402ms
Mana regen took 32,248ms
Mana regen took 32,143ms
Mana Regen 12
Mana Regen 12 - 313ms per tick - 31,300ms theoretical regen
Mana regen took 32,035ms
Mana regen took 32,077ms
Mana regen took 32,084ms
Mana Regen 11
Mana Regen 11 - 323ms per tick - 32,300ms theoretical regen
Mana regen took 37,908ms
Mana regen took 37,830ms
Mana regen took 37,933ms
Mana Regen 10
Mana Regen 10 - 333ms per tick - 33,300ms theoretical regen
Mana regen took 38,498ms
Mana regen took 38,470ms
Mana regen took 38,529ms
Mana Regen 9
Mana Regen 9 - 345ms per tick - 34,500ms theoretical regen
Mana regen took 38,486ms
Mana regen took 38,504ms
Mana regen took 38,546ms
Mana Regen 8
Mana Regen 8 - 357ms per tick - 35,700ms theoretical regen
Mana regen took 38,526ms
Mana regen took 38,501ms
Mana regen took 38,541ms
This does confirm your findings; it seems like MR has no effect past 12 at such high skills.
RunUO technical rant: I believe this is due to the regeneration timers having a tick priority of 50ms, making small differences in the time per tick (<50ms) have no effect. There are some other delays that factor in, such as the delay between a timer being queued and it being processed, which is why the plateaus don't occur exactly at 50ms intervals.
Below are tests repeated with the server-side mana regeneration tick priority adjusted to 10ms:
Mana Regen 18
Mana regen took 26,769ms
Mana Regen 17
Mana regen took 27,693ms
Mana Regen 16
Mana regen took 28,460ms
Mana Regen 15
Mana regen took 28,945ms
Mana Regen 14
Mana regen took 29,877ms
Mana Regen 13
Mana regen took 30,909ms
Now, the measured values get a lot closer to the theoretical ones. We probably need to make this very adjustment to the server core, however OSI verification is still needed to make sure their timers play nice at high skills. (Our behavior could still be accurate.)