There is no aparent diminishing returns on the critical hit statistic.
The formula for calculating critical hit % is:
Crit_Value * 0.0697 - 18.437

Maximum crit value (or, what I worked it out to be) is roughly 645.
341 native at L50, 304 from Daystar, Hero and Allag + Food. (Some pieces are lower ilvl but have materia slots. Mastr ACN ring.)

645*0.0697 - 18.437= 26.5%
You also don't sacrifice much MND with these crit intensive builds!

Enjoy critin', friend!