LOD Group and performance.

Hi there!

I have a question regarding the LOD system and I just can't find the answer. Is it more efficient to have 1 FBX file containing all my LODs event if it may induce that Unity will have to load 1500kb at all time to switch between the differents LODs or Is is better to affect to my LOD group different FBX (1 per LOD) has it would Load the mesh in memory only when needed?

PS: All my LODs use the same shader and the same textures.

Hope it's clear enough...

Thanks!