the “fresnel IOR” is just the refractive index of the material as used in the fresnel equations, n in the wikipedia entry. Metals are uniformly reflective, they do not exhibit fresnel reflections (the wikipedia entry on fresnel reflections notes an exception for conductive materials in general). That said, I have always been confused by the reflections IOR option in the standard material fresnel map. maybe it lets you have a different IOR for light transmission (“refraction”) and for reflection - which again doesn’t exist in the real world.
vray lets you do all kinds of things that don’t exist in the physical world. like having a different “highlight glossiness” and “reflection glossiness”. Maybe someday some materials scientist will invent a material that does have really high IOR.
So increasing the “fresnel IOR” beyond numbers that actual materials actually exhibit just plugs that number into fresnel’s equations, and it approximates a metal and you get a more uniform reflection.
if you want a more fresnel-like effect on a metallic reflection (that is, very reflective), try using the falloff map in the reflection color map, or use an angle-blend material with a more-reflective and less-reflective material.