platyping
Sleeping Dragon
While this is completely untested, it should do what you ask for. Well sort of. It has -25% production requirements. You could set it to return 80, which would be -20% requirements, which is the same as +25% production if you ignore the rounding errors.
Not really the same, but more or less. Reducing production cost itself = cheaper to hurry (Less gold, less population to sacrifice, less forests chopped), while increasing production pace has no effect on it.
It also gets escalated by other production modifiers.
For a 1000 cost, double production by marble, 100 normal rate:
Usual:
1000 / 200 = 5 turns
+25% production:
1000 / 225 = 5 turns
-20% cost
800 / 200 = 4 turns